[RelEng] Use new Tycho mojo to modify a composite P2-repository

[HannesWell]

Use the new modify-composite-repository Tycho mojo added via (respectively the back-port)

• Add mojo to modify a composite P2-repository eclipse-tycho/tycho#5228

This also uses the Maven profile already added in

• [RelEng] Simplify creation of release P2 repo and restore its mirrorsURL #3204

[HannesWell]

All testing was successful.
This is a pure RelEng change that's intended to be used immediately. Therefore I'm submitting this now despite we are currently in a freeze-period.

[HannesWell]

For the record: Because the new Tycho mojo works a little bit different than the ant-tasks worked before, it will automatically remove the categories sub-directory from the compositeContent.xml and therefore required

• [Build] Define IU categories directly within each build repository #3216