Only support the last 4 eclipse releases

[adangel]

Currently the plugin is still compiled against kepler, which is a Eclipse Release from 2013 (10 years old).

In order to make development easier, we'll only support the last 4 eclipse releases. This is at time of this writing:

• 2023-03
• 2022-12
• 2022-09
• 2022-06

The list of Eclipse Releases is here: https://wiki.eclipse.org/Simultaneous_Release

This means, that we don't need a toolchain anymore, we can just use Java17 to build and test the plugin. We can fix deprecation warnings. We can remove a couple of workarounds for bugs that have been fixed a long time ago.

The switch to PMD 7 seems like a good opportunity to make this change.