Bump the recommended-java-version to 21

[gastaldi]

@tqvarnst suggested bumping the recommended Java version to 21.

• Needed for https://issues.redhat.com/browse/QUARKUS-4111

[gsmet]

I think we should do that but I also think it's orthogonal to the issue.

My guess is that we should have a list of supported JDKs for each version together with a recommended version.

[gsmet]

@rsvoboda I think we should recommend Java 21 as default for Quarkus 3.8 too, right?

[gastaldi]

@gsmet We're discussing the options at https://quarkusio.zulipchat.com/#narrow/stream/187038-dev/topic/Platform.20metadata.20changes.

So far we've come up with a tested-java-versions array, which may contain the java versions that we have tested and know that work

[quarkus-bot]

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It should be safe to merge provided you have a look at the other checks in the summary.