Update version number in examples

Update example versions to 7.4.1

Enable unit testing of Java 20 and 21

Update ProGuardCORE version (Guardsquare#359)

Correct version of Java in release notes

Remove ClassPrinter debugging from ClassUsageMarkerTest

Add Kotlin 1.8 to release notes

Update GSON comments

Bump version to 7.3.0-beta2

Update version to 7.3.0-beta1