Use -Yfuture-lazy-vals for Scala 3 on the JVM

[adamw]

Adds -Yfuture-lazy-vals to Scala 3 on the JVM, so that JVM 25 no longer warns about a terminally deprecated API. Same change as softwaremill/sttp#2996.

The flag needs the VarHandle API, so it comes with -java-output-version. That option also decides which JDK APIs the compiler sees, not just the class file version, so the number has to match what a module actually requires:

• 11 by default. Scala 3 artifacts now need Java 11; Scala 2 artifacts are unchanged and still target Java 8.
• 21 for nettyServerSync and nimaServer, which need JDK 21 anyway — the first through ox, the second through Helidon Nima.

The Scala 2.13 row of nimaServer is untouched and still targets Java 8, as before.

The flag only exists in the 3.3 LTS line, so it's tied to scala3 rather than to "any Scala 3". That leaves out openapiCodegenSbt, which builds with 3.7.4 and would fail on an unknown option. From 3.8 on the same lazy val encoding is the default, so the flag can go once the main Scala 3 version moves there.

Checked: compileScoped 3 JVM passes, with and without ONLY_LOOM, core3/test passes, and emitted class files are version 55 and 65 as expected.