chore(ci): add merge_group trigger to required workflows

[ARR4N]

Why this should be merged

Allows ruleset-required workflows to be triggered by a merge queue, not just the PR for the merge. Although main isn't a particularly "busy" branch, a merge queue will ensure that CI passes on the exact version of code that is about to be merged; i.e. (from the docs):

The merge queue provides the same benefits as the Require branches to be up to date before merging branch protection, but does not require a pull request author to update their pull request branch and wait for status checks to finish before trying to merge.

How this works

Adds merge_group workflow trigger with recommended type.

How this was tested

N/A (worst-case it has to be reverted and another PR is temporarily blocked).