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