Bump JDK version requirement in Gradle and CI

[mjameswh]

What changed

• Uniformly use Java 23 for all gradle/tooling execution in both Gradle and GHA workflows.
• Use gradle's toolchains to ensure runtime tests are run against the correct version of Java.
• Add a gradle property testJavaVersion that can be used to select the version of Java to use to run tests. For example: ./gradlew test -PtestJavaVersion=11.
• Remove the com.palantir.git-version gradle plugin. Its features are actually no longer used.

Note

• We unfortunately can't actually run tests against Java 8 because Mockito 5 requires at least Java 11. Compiling the code with --release 8 still provides a reasonable level of confidence that the code is safe to run in a JRE 8 environment.