Generate the google-java-format default from the version catalog

[builtbyjuls]

Closes #2705.

Summary

• Add gradle-buildconfig-plugin to the library build.
• Generate a package-private BuildConfig.VERSION_GJF from the google-java-format version catalog entry.
• Use the generated value for the JVM 21+ default.
• Keep the JVM 17 default and JVM minimum-version constraints explicit because they represent compatibility rules rather than the latest dependency version.

Verification

• ./gradlew :lib:clean :lib:generateBuildConfigClasses :lib:compileJava --stacktrace
• ./gradlew :lib:compileJava :testlib:test --tests com.diffplug.spotless.java.GoogleJavaFormatStepTest spotlessCheck --stacktrace

The focused test run executed 13 tests: 10 passed and 3 JVM-specific tests were skipped.

No CHANGES.md entry: this changes build maintenance only and does not change the current formatter default or user-facing behavior.

[Goooler]

Needs to check the other versions as well, replace all related usages.

[builtbyjuls]

Thanks. To confirm, should I generate constants for every catalog-backed formatter/runtime default in lib and replace their usages across lib and the plugins, while keeping compatibility thresholds and intentionally older compile pins hardcoded?

I found four catalog/default mismatches: Gson 2.14.0/2.13.2, Jackson YAML 2.20.1/2.14.1, Cleanthat 2.24/2.25, and Palantir 1.1.0/2.80.0. Including those would change or downgrade current defaults. Should this PR reconcile those too?