Move to JDK 25 #86

Move every module to JDK 25 at the same time: the reactor, including ng-core and ng-template-parser, plus ng-testapp. Every module stays on one Java level; no module is kept on an older one.

• In the root pom.xml, replace <source>21</source><target>21</target> with <release>25</release>. With release, the compiler also rejects calls to APIs the target JDK doesn't have.
• ng-testapp has its own maven-compiler-plugin block pinning 21 and needs the same change.
• Update the "Java 21 or later" requirement in the README.

Consumers then need a JDK 25 runtime. That includes WO/wonder-slim projects using ng-template-parser and the Parslips Eclipse plugin, so check those before releasing.