Add CI build using JDK 25

[Marcono1234]

Purpose

Add CI build using JDK 25, so that we can be sure Gson works with JDK 25

Supersedes #2847

Description

However, because building with JDK 25 does not fully work yet, mainly due to issues with ProGuard, still disallow building with JDK 25 by default. This way it fails fast for users locally instead of causing confusing failures later on.

Note that there are a few warnings related to Unsafe usage by Maven (respectively its dependencies) and by protobuf-java.

And also one test is causing warnings:

[INFO] Running com.google.gson.typeadapters.UtcDateTypeAdapterTest
WARNING: Use of the three-letter time zone ID "NST" is deprecated and it will be removed in a future release
WARNING: Use of the three-letter time zone ID "AST" is deprecated and it will be removed in a future release
WARNING: Use of the three-letter time zone ID "MST" is deprecated and it will be removed in a future release

(that test uses all time zones provided by TimeZone.getAvailableIDs())

But besides that the build and Gson itself seems to work fine with JDK 25.

[Marcono1234]

For simplicity I reused the existing excludeTestCompilation property which we are already using for excluding JDK 17+ tests on JDK 11.

A slightly cleaner solution might be to adjust the maven-surefire-plugin configuration instead, to exclude that test. But I am not sure if that is worth it, given that hopefully soon in the future ProGuard supports JDK 25.

[eamonnmcmanus]

Seems very reasonable. Thanks for doing this!