Review workflows that only run on pull requests against main

Summary

Several workflows across the MDN org restrict pull_request triggers to branches: [main], e.g. PR Test in mdn/translated-content:

on:
  pull_request:
    branches:
      - main

This prevents the workflow from running on stacked PRs (PRs that target another PR's branch instead of main). Since these workflows typically compute their diff from the PR's own base and head SHAs, the filter seems unnecessary and may be unintentional.

Example

mdn/translated-content#38394 targets another branch, so PR Test never ran, and hence PR review companion never deployed a preview. Lint content (no branch filter) ran as expected.

Proposal

Review the pull_request triggers across MDN repos (mdn/content, mdn/translated-content, mdn/fred, mdn/rari, mdn/yari, mdn/workflows, ...) and drop the branches: [main] filter where the workflow does not actually depend on the base branch.