Rename release migration script

Version Control Information

Source branch: bruno.sc/rename-release-migration-script Target branch: devel

Commits:

* Rename release migration script

  `actions/migrate-cluster-repo.sh` has been provided to migrate a cluster repo
   after upgrading to the release v9 series.
  With upcoming release series and v10 already migrations won't happen in the
   cluster repo alone hence the release migration script's name needs to reflect
   just that.

  In this process "migration" has been disambiguated to "release migration".

  Resolves: #750

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Closes: #750 (closed)
After: !1792 (merged)

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• Documentation has been updated, if necessary
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• Release note file in RST format added in latest commit