✨ [#4215] Add delete_old_emails task to celery beat schedule

[stevenbal]

Closes #4215

Changes

• Add delete_old_emails task to celery beat schedule

Checklist

Check off the items that are completed or not relevant.

• Impact on features

  • Checked copying a form
  • Checked import/export of a form
  • Config checks in the configuration overview admin page
  • Problem detection in the admin email digest is handled

• Release management

  • I have labelled the PR as "needs-backport" accordingly

• I have updated the translations assets (you do NOT need to provide translations)

  • Ran ./bin/makemessages.sh -> is being handled in 🌐 Re-generate translation catalogs #4239
  • Ran ./bin/compilemessages_js.sh -> is being handled in 🌐 Re-generate translation catalogs #4239

• Commit hygiene

  • Commit messages refer to the relevant Github issue
  • Commit messages explain the "why" of change, not the how

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.16%. Comparing base (93e9ba2) to head (b66a8ce).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4245   +/-   ##
=======================================
  Coverage   96.16%   96.16%           
=======================================
  Files         732      732           
  Lines       23445    23445           
  Branches     2737     2737           
=======================================
  Hits        22546    22546           
  Misses        637      637           
  Partials      262      262           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.