CI: Generate GitHub releases

* Increase the checkout depth from 1 to 30 to handle reno needing to be
  able to find all relevant tags (current and current -1) and also pull
  the tags
* Setup Python to run reno
* Generate a reno report for the given tag and then modify the report to
  strip out all the extra garbage that is added by the report process to
  get it into a valid markdown file and not RST file
* Use `gh` cli tool to generate the release notes using the
  auto-changelog generation plus the reno release notes

Issue: RELENG-4975
Change-Id: Ie05579dfd757e7d3be7169da987bf67229f4f818
Signed-off-by: Andrew Grimberg <agrimberg@linuxfoundation.org>