openjdk version upgrade (to 21)

[alexeyqu-fb]

Currently we use javaDockerContainer=openjdk:8-jdk which is slightly outdated.
I wonder if anyone has objections agains upgrading it to a later version (e.g. 11 or even 17).

cc @seehamrun

[janhoy]

We now use javaDockerContainer=openjdk:11. Should probably bump to an even newer version though..

[alexeyqu]

It was upgraded to 11 in #1114, yes.

[alexeyqu]

Looks like the current good LTS baseline is JDK 21, https://openjdk.org/projects/jdk/21/

[lisad]

OpenJDK now has version 21 and 25, both long-term supported releases. need to identify who can work on bringing this up to date.