Extend release notes script to sweeps

Add a --sweep mode to create "release notes" (MR diffs) for our daily master merges. The main change is to add a new template for merge descriptions and use git log HEAD^..HEAD to retrieve the diff.

See !61782 (merged) for an example.

cc @tadej @emoyse @abarton @wlampl @tsulaia @elmsheus @nstyles @jcatmore

TODO:

• Update https://atlassoftwaredocs.web.cern.ch/guides/release_coordinators