SpringBoot Use Bellsoft JDK 21 image when Java 21 is out of early access

LoaderIntegrationTests uses openjdk:21-ea-jdk. We should change that to use the bellsoft/liberica-openjdk-debian:21 when Java 21 is out of early access and Bellsoft published a docker image for it.

Comment From: wilkinsona

With CI for Java 21 across all maintained branches, this change now applies to 2.7.x onwards.

Comment From: mhalbritter

It's available: https://hub.docker.com/r/bellsoft/liberica-openjdk-debian/tags?page=1&name=21