[2.9.x] Test with Java 21 instead of 11

[mkurz]

Look at

• [2.9.x] Build only changed examples and do it parallel (backport #559) by @ihostage #582
• [3.0.x] Build only changed examples and do it parallel (backport #559) by @ihostage #581

These are more or less identical pull requests. However, if you look closer at #581 you will see it changed java 11 to 21.
Why this did not happen in #582 I don't know. So I guess 1) either it was forgotten to change it in #582 too or 2) it was a mistake to change it to 21 in #581.

Anyway, I think it's a good idea to just run against 17 and 21 and just drop 11 now (even though Play 3.0 and 2.9 still support 11). These are just example projects and some dependencies already require Java 17 (like flyway #780) - I think it's a good idea to show examples with latest dependencies even though they can not be used with Java 11.

[ihostage]

I just thought we should check an examples for minimal supported Java versions. 🤔
What do you think about check two version: minimal supported and latest LTS?
In 2.9.x case it will be 11 and 21.

[mkurz]

We can do that as well of course, but then we need to pin some dependencies, like flyway, which in its latest version(s) requires Java 17.