Return a SortedSet in EEManager's getSupportedExecutionEnvironments()

[HannesWell]

What it does

Similar to https://github.com/eclipse-jdt/eclipse.jdt.core/pull/2770, return a SortedSet in IExecutionEnvironmentsManager.getSupportedExecutionEnvironments().

And don't reverse the EE order to be consistent with existing methods. As soon as Java-21 is used, one can just use the reversed() method inherited from SequencedSet to get the supported EE's in descending order.

Author checklist

• [ ] I have thoroughly tested my changes
• [x] The change is following the coding conventions
• [x] I have signed the Eclipse Contributor Agreement (ECA)

[HannesWell]

@iloveeclipse, can you take a look at this? I know this is very late, but I hope not too late (I actually wanted to propose this earlier, but forgot it). Sorry for the trouble, but I hope a better return type in an API probably supported for a long time is worth it.

I have already prepared the corresponding change in jdt.ui in https://github.com/eclipse-jdt/eclipse.jdt.ui/pull/1600, which has to be submitted together with this change.

[HannesWell]

Thank you Andrey!