Remove support for OpenJDK 11 on the server side in Keycloak 22

[Stian Thorgersen]

We are currently supporting OpenJDK 11 on the server-side, and planning to support OpenJDK 17 from 20 onwards.

With new Java releases coming more and more frequently I propose we aim to support only two versions at a given time. The latest LTS and the latest non-LTS.

This would mean dropping support for OpenJDK 11 shortly after supporting OpenJDK 17.

https://github.com/keycloak/keycloak/issues/15014