Add new workflow to fixup backports for release notes (backport #3252) (#3253)

* Add new workflow to fixup backports for release notes (#3252)

The Mergify-generated backports do not currently include the information
needed by the release notes generation automation. This new workflow
will copy such information over to backport PRs.

(cherry picked from commit 86ccd1b2)

* Fix backport-fixup for labels with spaces (#3255)

(cherry picked from commit 8f7e7e68

)

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Co-authored-by: Jack Koenig <koenig@sifive.com>