fix(1694): changelog_merge_prerelease not working on cz bump

[zimmermannJakob]

Description

This PR solves issue #1694. This PR is still in draft, as I want to check whether the changes make sense conceptually and manual tests are still pending.

Checklist

• I have read the contributing guidelines

Code Changes

• Add test cases to all the changes you introduce
• Run poetry all locally to ensure this change passes linter check and tests (Some error out on mac. I will investigate if they fail in ci)
• Manually test the changes:
  • Verify the feature/bug fix works as expected in real-world scenarios
  • Test edge cases and error conditions (I thought about breaking changes and starting an empty changelog with prereleases.)
  • Ensure backward compatibility is maintained
  • Document any manual testing steps performed
    - [ ] Update the documentation for the changes (I do not think this applies. It works now as documented.)

### Documentation Changes

- [ ] Run poetry doc locally to ensure the documentation pages renders correctly
- [ ] Check and fix any broken links (internal or external) in the documentation

When running poetry doc, any broken internal documentation links will be reported in the console output like this:

INFO    -  Doc file 'config.md' contains a link 'commands/bump.md#-post_bump_hooks', but the doc 'commands/bump.md' does not contain an anchor '#-post_bump_hooks'.

Expected Behavior

See issue #1694.

Steps to Test This Pull Request

1. build the changes on the branch. poetry build.
2. Take the resulting .wheel file from dist/
3. Install the file in a separate python environment:

• python3 -m venv .venv && source .venv/bin/activate
• verify env: which pip
• pip install commitizen-4.10.0-py3-none-any.whl

4. Go through the steps in the issue description and try reproducing the bug.

Additional Context

Known issue: When the changelog only contains pre-releases, the old behavior can still be observed. (Fixed)

[codecov]

Codecov Report

❌ Patch coverage is 97.36842% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 98.66%. Comparing base (120d514) to head (73aa8a3).
⚠️ Report is 860 commits behind head on master.

Files with missing lines	Patch %	Lines
commitizen/changelog_formats/base.py	94.11%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1700      +/-   ##
==========================================
+ Coverage   97.33%   98.66%   +1.32%     
==========================================
  Files          42       60      +18     
  Lines        2104     2691     +587     
==========================================
+ Hits         2048     2655     +607     
+ Misses         56       36      -20     

Flag	Coverage Δ
unittests	98.66% <97.36%> (+1.32%)	⬆️

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