[37] Convert into a plain Maven module

[pcdavid]

• There is now a single Maven module; the test code is directly in src/test/java.
• Removed all Eclipse/PDE-specific settings.
• Removed the Eclipse-specific code and tests which depended on it.
• There is still an Eclipse project, but it is configured as an M2E project, node a plug-in (PDE).
• The resulting JAR should still be OSGi-compliant, the produced MANIFEST.MF is the same as before (it may not be in sync with the POM version though).
• Moved compiler settings to Java 17.
• Removed some unused dependencies.
• Added mvn wrapper and use it in the workflow to ensure a recent Maven version.
• The resulting project can be imported in an Eclipse workspace and the test suites are green both when run from inside Eclipse and via mvn verify: