[Feature] Update to Java/JDK 21

[sventorben]

Is there an existing feature request for this?

• I have searched the existing issues

Is your feature related to a problem? Please describe.

KC supports Java 21 since version 25.0.0.

Upate to Java 21 once KC does no longer support Java 17.

[ahus1]

Removing support for JDK 17 in Keycloak is currently planned for Keycloak 27 (due March 2026)
keycloak/keycloak#31101