Run release-notes job only when tag is created

Médéric de Verdilhacrequested to merge
release_note_at_tag_push_only into main

Add condition $CI_PIPELINE_SOURCE == "push" in release-notes job rules, in order to make sure job are only launched at tag creation and not on subsequent pipelines run using a tag ref.

Also create a .release-notes-rules template to set rules once and reuse across jobs.

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