fix multiple workspace warnings

[Bananeweizen]

All remaining workspace warnings are now coupled to bad versioning of dependencies (e.g. Xtend last() method, deprecated PDE methods etc.)

[Bananeweizen]

@haubi can you please review this one? it doesn't add any version restrictions or similar, but just removes warnings without functional changes

[haubi]

Regarding the Xtext warnings, I've added a comment for why they are there - the difference is in the Xtext generated code.
The other 2 commits I've picked into master, which now requires Eclipse 2022-09 and Java 17, with the idea in mind to create the 0.8.0 release (maybe in January).
For Eclipse 2024-09 and Java 21 I've created an eclipse-2024-09 branch for now, should become the 0.9.0 release afterwards.

IIRC, we have talked about declaring dependency versions.
Although I have some ideas about it: What exactly do you mean?
Is there any tooling that helps in declaring (and updating) the dependency versions?