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

.gitignore

@@ -8,4 +8,10 @@ out/
 
 # OS
 .DS_Store
-Thumbs.db
+Thumbs.db
+
+# Nextcloud sync
+.sync-exclude.lst
+
+# Build artifacts (kept on disk for the submission ZIP, not in git)
+release/*.jar