Modernize the toolchain: Scala 2.13.16, JDK 21, Apache Pekko, and wider PostgreSQL support

[GeekOfWires]

Process Proposal

This PR exists primarily to give the project visibility into the full process of making
the changeover, shown as one continuous path rather than a polished end state. It is
admittedly larger than a comfortable single merge, and would reasonably be broken up
before landing.

It is presented here as a single pull request because the changes are interdependent:
getting from Akka-runnable on the earlier JDK to Pekko-runnable on the latest JDK took
several linked steps — toolchain bump → Akka→Pekko migration → JDK 21 → the runtime and
dependency fixes that only surface once the server actually boots — and no single step is
runnable on its own without the others. Keeping them together documents the real path taken
and lets reviewers see exactly what each hop required, before deciding how to slice it for
merge.

Summary

This branch modernizes the build/runtime stack in small, individually-reviewable phases,
each in its own commit. Nothing in game logic or the network protocol changes — this is
purely infrastructure. It compiles green, runs on JDK 21, and introduces zero test
regressions (evidence below).

#	Commit	What it does
0	Phase 0: bump Scala, sbt, and low-risk dependencies	Scala 2.13.10 → 2.13.16, sbt 1.8.2 → 1.10.7, plus low-risk dependency bumps
1	Phase 1: migrate Akka 2.6.20 -> Apache Pekko 1.1.x	akka.* → org.apache.pekko.* (pekko-actor 1.1.3, pekko-http 1.1.0), akka.conf → pekko.conf, config namespace akka { } → pekko { }
2	Phase 2: move build/runtime/CI to JDK 21	Dockerfile builder + eclipse-temurin:21-jre runtime, CI JVM → temurin 21, bcprov-jdk18on 1.78.1
3	Phase 3: widen PostgreSQL version support	Flyway 10 (flyway-core + flyway-database-postgresql 10.20.1), postgresql JDBC 42.7.4; CI matrix now proves migrations across 12.8 / 16 / 17 without dropping the current version
–	Add missing Default import in LocalActionTest	Fixes a pre-existing test-compile break on master (see below)
–	Enable server runtime on JDK 21: Kamon 2.7.7, Kanela 2.1.1, logback 1.5.18	Runtime-only fixes uncovered by actually launching the server on JDK 21 (see below)
–	Fix Docker image tag; run compose stack on JDK 21 / Postgres 18	The prior JDK-21 sbt image tag isn't published on Docker Hub; also moves docker-compose off the stale JDK 8 image

Why

• License future-proofing (the main motivation). Akka moved to the BSL 1.1 license at
  2.7.0; the project is pinned to the last Apache-2.0 Akka (2.6.20). Apache Pekko 1.x is the
  community fork of Akka 2.6.x under Apache-2.0 and is a near drop-in (namespace change only).
  Migrating now keeps the actor layer on a permissively-licensed, actively-maintained base and
  unblocks future actor/stream/HTTP upgrades.
• Supported toolchain. JDK 21 (current LTS) + Scala 2.13.16 + sbt 1.10.7 are all
  actively supported; the previous sbt 1.8.2 / Scala 2.13.10 combination will not even start
  on JDK 21.
• PostgreSQL headroom. Flyway 10 + a current JDBC driver let operators run newer
  PostgreSQL majors (verified up to 18) while the CI matrix keeps proving the current 12.8
  still migrates — no forced deprecation.

Verification

• sbt clean test compiles all source roots and runs the full suite on JDK 21.
• The stack was run end to end via docker-compose: the server boots on JDK 21, Pekko's
  Slf4jLogger starts, Flyway applies all 17 migrations to a fresh PostgreSQL 18.4
  database, and the login server listens. (Flyway 10.20.1 emits only a soft "newer than
  tested" warning on PG 18 and migrates successfully.)

Test status — please read

The suite currently reports 192 pass / 38 fail (scalatest) + 3 specs2 failures. These are
not introduced by this branch — they reproduce identically on today's master. I confirmed
this with a full attribution matrix:

Config	Scala	Actor lib	JDK	Result
master (unchanged)	2.13.10	Akka 2.6.20	17	192 / 38 + 3
this branch, Pekko reverted	2.13.16	Akka 2.6.20	17 & 21	192 / 38 + 3
this branch	2.13.16	Pekko 1.1.3	21	192 / 38 + 3

Same count, same suites across every axis → the Pekko/JDK/Scala changes flip none of them.
The failing suites are timing-sensitive objects.* ActorTests (the ActorTest base class
documents this flakiness) tied to the message bundling introduced in #1379; a same-toolchain
Akka baseline reproduces them identically. Happy to share the full per-suite breakdown.

Separately, the Add missing Default import in LocalActionTest commit fixes a spot where
master's test sources don't currently compile (LocalActionTest references Default.GUID0
without importing net.psforever.objects.Default).

Deliberately conservative / follow-ups

• pekko-stream / pekko-http / cluster modules are kept for parity but have no direct
  imports yet — safe removal candidates in a follow-up.
• sbt-scalafix is temporarily disabled (semanticdbEnabled := false) because 0.10.4 pins a
  semanticdb with no 2.13.16 build; re-enable alongside a scalafix bump.
• Kamon's kamon-pekko instrumentation and a jasync/Quill bump are left for a later pass.

Reviewing commit-by-commit is recommended — each phase stands on its own and can be split into
separate PRs if that's preferred.

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Prepared with assistance from Claude (Opus 4.8); commits carry Co-Authored-By trailers accordingly.