Remove deprecations preparing for v5

[mauvilsa]

What does this PR do?

Removes all features deprecated during v4 and makes the announced v5 behaviors the default. The _deprecated module and its test file are deleted, and the documentation drops the notes that described the transitional behaviors.

What was removed or changed, and how to update code, is already in the v5.0.0 section of the changelog and in the migrate to v5 guide, so it is not repeated here.

Not covered there, since it is not user facing: internal code paths that the removals made unreachable are deleted as well, e.g. the list handling in replace_unset and expand_dict, only needed for behaviors that no longer exist.

Before submitting

• Did you read the contributing guideline?
• If you used a coding agent, did you fully understand and validate all generated code and ensure it follows the contributing guidelines?
• Did you update the documentation? (readme and public docstrings)
• Did you write unit tests such that there is 100% coverage on related code? (required for bug fixes and new features)
• Did you verify that new and existing tests pass locally?
• [n/a] If this is a bug fix, did you verify that the tests fail without the code fix?
• [n/a] Did you make sure that all changes preserve backward compatibility?
• Did you update the CHANGELOG including a pull request link? (not for typos, docs, test updates, or minor internal changes/refactors)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (e063051) to head (45036f5).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##              main      #969    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files           28        27     -1     
  Lines         9388      8729   -659     
==========================================
- Hits          9388      8729   -659     

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
1 Accepted issue

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud