Revisit build to promote multi-release jar support

[bclozel]

As of #34220 the build is using a Java 23 toolchain to compile classes with the relevant "-release" compilation parameter to properly target the Java 17 baseline or higher for multi-release Jars.

This issue is about refactoring our build to fully enforce this and prepare for Java24 specific code:

• remove our custom toolchain configuration for testing
• use a custom multi-release jar that uses the "-release" parameter instead of the language level, assuming that the main toolchain is always configured with a Java version that is higher than all multi-release jar specific variants