:boom: Drop legacy formio translation converter

[sergei-maertens]

Related to #3283

Per the deprecation warning we drop support for exports containing the legacy translation store. This also gave the opportunity to clean up some known technical debt for the search strategies, and now that they're stable, they are also documented as available testing tool.

Checklist

Check off the items that are completed or not relevant.

• Impact on features

  • [x] Checked copying a form
  • [x] Checked import/export of a form (breaks legacy imports, but that's deliberate)
  • [x] Config checks in the configuration overview admin page
  • [x] Problem detection in the admin email digest is handled

• Release management

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

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

  • [x] Ran ./bin/makemessages_js.sh
  • [x] Ran ./bin/compilemessages_js.sh

• Commit hygiene

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

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 96.57%. Comparing base (5042ea1) to head (595c8ba). Report is 6 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #4828 +/- ## ========================================== - Coverage 96.57% 96.57% -0.01% ========================================== Files 748 747 -1 Lines 25448 25400 -48 Branches 3369 3355 -14 ========================================== - Hits 24576 24529 -47 Misses 608 608 + Partials 264 263 -1 ```

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