Update to Java 21 and Gradle 8

[jra-se]

No description provided.

[github-actions]

❌ Changes break the MontiVerse

Project	Status
umlp_experiments	❌
artifacts	❌

details

Project	Status
xml	✅
montiarc	✅
umlp_experiments	❌
umlp_tool	✅
sd	✅
montifun	✅
fd	✅
automaton	✅
sc	✅
guidsl	✅
artifacts	❌
ocl	✅
od	✅
ucd	✅
langexperiments	✅
json	✅
cd2gui	✅
mlc	✅
cd4analysis	✅

The MontiVerse is a collection of (internal and public) language projects.