Add django 4.2, 5.2 and 6.0 support, drop older versions

[nezhar]

This pull request updates the project to support newer versions of Django and Python, and removes compatibility with older versions. It also modernizes the CI workflow and cleans up legacy code and references. The most important changes are grouped below.

Version Support Updates:

• Added support for Django 4.2, 5.2, and 6.0, and Python 3.11, 3.12, 3.13, and 3.14. Dropped support for Django 3.2, 4.0, 4.1, and Python 3.7, 3.8, and 3.9. Updated setup.py, CHANGELOG.md, and README.md to reflect these changes.
• Updated migration and test files to remove compatibility code for older Django/Python versions and to use newer APIs (e.g., timezone imports, test assertions).

CI Workflow Modernization:

• Updated .github/workflows/tests.yml to use newer versions of GitHub Actions, Codecov, and Python/Django matrix. Removed experimental matrix and updated install commands for Django.

Documentation Improvements:

• Updated README.md to clarify supported versions and reference latest Django documentation.

Code Cleanup:

• Removed legacy middleware compatibility code for old Django versions in django_userforeignkey/middleware.py.
• Removed deprecated USE_L10N setting from tests/test_project/settings.py.

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.61%. Comparing base (9a85066) to head (4666415).
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Additional details and impacted files

@@            Coverage Diff             @@
##           master      #18      +/-   ##
==========================================
+ Coverage   90.03%   90.61%   +0.57%     
==========================================
  Files          10       10              
  Lines         281      277       -4     
==========================================
- Hits          253      251       -2     
+ Misses         28       26       -2     

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