Add labels to monthly release version MRs

This merge request attempts to address the feedback I had in technical-writing#615 (comment 957311639) regarding missing labels in the auto-generated version-specific MRs that we auto-generate during every docs release process.

I have no idea how to test this change, unfortunately. I'm hoping either @sarahgerman or @axil will know.

Related items

• Related to Docs release 15.0 retrospective (technical-writing#615 - closed) where the need was identified.