Build on libmongoc 2.0–2.2 (Ubuntu 26.04) and drop the misleading apt hint

[taplin]

Two Linux installation problems, intended for a 4.0.2 patch release.

1. 4.0.1 doesn't compile against libmongoc 2.2 (Ubuntu 26.04 LTS)

Ubuntu 26.04's libmongoc-dev is 2.2.2. 4.0.1 calls bson_append_array_unsafe_begin, which libbson only added in 2.3.0; the same release deprecated bson_append_array_begin. A small PerfectCBSON shim now uses BSON_CHECK_VERSION(2, 3, 0) to call whichever this libbson has. The result builds warning-free on 2.3+ and still works on 2.0–2.2.

2. The apt install hint was misleading

When libmongoc is missing, SwiftPM printed apt-get install libmongoc-dev from the package's providers:. On Ubuntu 24.04, the base of the official swift:*-noble images, and on Debian 13, that installs libmongoc 1.x, which 4.x can't use. SwiftPM's hint can't vary by release, so the apt provider is removed and the Homebrew one, which is correct, stays. The READMEs (English and Chinese) now list which distributions' packages work:

Distribution	libmongoc-dev
Ubuntu 26.04 LTS+	2.2+	apt works
Debian forky / sid	2.5	apt works
Ubuntu 24.04, Debian 13 and older	1.x	build from source

CI

There's a new linux-apt job: the swift:6.4-resolute container with libmongoc from plain apt (2.2.2). It tests the oldest supported libmongoc and the README's apt instructions.

Testing

Full suite (36 tests):

• libmongoc 2.2.2 on Linux, built from the release tarball; this PR's new CI job covers the apt package itself;
• libmongoc 2.5.5 on macOS (Xcode 27.0) and on Linux.

No warnings on 2.5.5.

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