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/pom.xml

@@ -9,8 +9,10 @@
     <version>1.0-SNAPSHOT</version>
 
     <properties>
-        <maven.compiler.source>25</maven.compiler.source>
-        <maven.compiler.target>25</maven.compiler.target>
+        <!-- Java 21 (LTS): the newest feature actually used is List.getFirst()
+             (SequencedCollection, Java 21). Building against an LTS keeps the
+             project compilable on any current JDK. -->
+        <maven.compiler.release>21</maven.compiler.release>
         <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
 
         <jackson.version>2.18.2</jackson.version>