This commit revisit the build configuration to enforce the following:
* A single Java toolchain is used consistently with a recent Java
version (here, Java 23) and language level
* the main source is compiled with the Java 17 "-release" target
* Multi-Release classes are compiled with their respective "-release"
target. For now, only "spring-core" ships Java 21 variants.
Closes gh-34507
This commit moves the checkstyle conventions from the build.gradle
script to a buildSrc convention, ensuring that the same configuration is
applied to all checkstyle tasks.
See gh-30339
