For SDK tests:
- Defined dedicated compliance.jre from 21 to 23, removed unused
- SDK tests will run on Java 21 tests for 1.8,11,17,20,21 JLS
- Changed compliance.jre.17 to 1.7,1.8,11,17 (added 1.7, omitted 16)

For Jenkins (github validation):
- Changed compliance to 1.8,11,17,20,21 (replaced 19 with 20)

Fixes https://github.com/eclipse-jdt/eclipse.jdt.core/issues/1605