[CI] GitHub Actions fail because of Java 21

[bivashy]

Description

Currently, the latest GitHub actions fail and don't return any useful information. For example, #665 fails with an error:

Failed to execute org.apache.maven.plugins:maven-compiler-plugin:3.11.0:compile (default-compile) on project cobalt: Fatal error compiling: error: release version 21 not supported -> [Help 1].

And other actions have the same error.

This could be fixed by simply updating the Java version in the build workflow.