fix: Avoid duplication of placeholder checks for locked versions

[fsbraun]

Description

djangocms-versioning adds default_checks to placeholders to avoid changing content of published objects. The checks for locked versions were added for every versioned model, but only need adding once.

This PR moves adding the check for locked versions from cms_config.py to apps.py, where it is only done once.

Related resources

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Checklist

• [x] I have opened this pull request against master
• [x] I have added or modified the tests when changing logic
• [x] I have followed the conventional commits guidelines to add meaningful information into the changelog
• [x] I have read the contribution guidelines and I have joined #workgroup-pr-review on Slack to find a “pr review buddy” who is going to review my pull request.

[codecov[bot]]

Codecov Report

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

Project coverage is 91.60%. Comparing base (f90c5b2) to head (0094662). Report is 12 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #393 +/- ## ========================================== + Coverage 90.88% 91.60% +0.71% ========================================== Files 72 72 Lines 2546 2573 +27 Branches 361 366 +5 ========================================== + Hits 2314 2357 +43 + Misses 168 153 -15 + Partials 64 63 -1 ```

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