[workflows] Remove fix-version-gaps (elastic#190721)

This workflow was intended to automatically update labels if a pull
requests version targets were out of date at the time of merge (i.e. a a
pull request labeled with 8.14 as a target branch, is now out of date
after the global version is bumped to 8.16, will add the 8.15 label)

We don't add target versions anymore pre-merge except in rare cases like
documentation updates. This last time this workflow was run was 2.5
years ago.