ci: use wp-cli i18n command instead of pb-cli

[greatislander]

Partial resolution to https://github.com/pressbooks/private/issues/910. In addition to replacing the wp pb make-pot command with wp i18n make-pot, this PR also replaces the update-translations.yml workflow's use of gettext with the wp i18n make-mo command, which does the same thing.

I've commented out the step which autocommits changes. Instead, when these workflows run if they result in any changed files, they will open a PR. Devs can merge if it looks good. In future, you may wish to switch back to autocommit.

[codecov-commenter]

Codecov Report

Merging #335 (90bff6e) into dev (2a0ff78) will not change coverage.
The diff coverage is n/a.

❗ Current head 90bff6e differs from pull request most recent head 2a61733. Consider uploading reports for the commit 2a61733 to get more accurate results

@@          Coverage Diff          @@
##             dev    #335   +/-   ##
=====================================
  Coverage   0.00%   0.00%           
=====================================
  Files          7       7           
  Lines        590     590           
=====================================
  Misses       590     590           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 2a0ff78...2a61733. Read the comment docs.

[SteelWagstaff]

👍