Fully use Gradle's Java Toolchain feature once it supports JDK patch versions

[jjohannes]

We currently cannot use this as there is no guarantee for selecting an exact JDK version. We need that to make sure that builds are reproducible. See:

• Make reproducible builds the default gradle/gradle#28806 (comment)
• [Maybe] Ensure consistency for module-info.class files gradlex-org/reproducible-builds#8
• Allow to specify minor, hotfix and build number for java toolchain gradle/gradle#16628
• Introduce 'implementationVersion' to JavaToolchainSpec gradle/gradle#32522