Add preview images for jdk21

[nbr23]

Add jdk21 preview images

This is something that's been discussed here #587 in an effort to have alpine based arm64 images for the inbound agent.

Testing done

Built locally

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[gounthar]

That's quite an impressive first contribution! Congratulations!

[dduportal]

That is a nice first contribution! Thanks!

I understand that adding 2 new Dockerfiles is related to the way we "retrieve" the JDK on the existing one, which is currently incompatible with JDK21 as Temurin does NOT provide the Docker images with ea JDK21.

=> It is fine to keep this and we'll "merge" the JDK21 Dockerfiles with the existing ones either by changing the default JDK retrieval method (cc @gounthar as I remember you tried this kind of changes in jenkinsci/docker as an experimen t a few months earlier) to use the ZIP installed, OR by waiting for the images publications.