build: target Java 21 (LTS) instead of Java 25

maven.compiler.source/target were set to 25, but the newest language feature
actually used is List.getFirst() (SequencedCollection, Java 21). On a JDK 17 or
21 the project would not compile at all.

- Use maven.compiler.release=21 instead of the source/target pair
- README: update prerequisites, tech stack and test count
- .gitignore: ignore the Nextcloud sync file and release/*.jar

Semesterprojekt/.gitignore

@@ -38,12 +38,6 @@ build/
 ### Mac OS ###
 .DS_Store
 
-### Brainstorming visual companion ###
-.superpowers/
-
-# Planning specs & implementation plans (kept locally, not tracked)
-docs/superpowers/
-
 # Local save games and settings
 saves/