refactor(config)!: remove deprecated .proselintrc

[Nytelife26]

Brief

We have been carrying this deprecation notice around for several years and several releases. Getting rid of the old bare .proselintrc files in favour of .proselintrc.json is long overdue.

Changes

• Remove deprecation warning and read system for old configuration files
• Remove references to legacy configuration in README

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.63%. Comparing base (d19d1c8) to head (f408607).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1471      +/-   ##
==========================================
+ Coverage   92.56%   92.63%   +0.06%     
==========================================
  Files         100      100              
  Lines        1022     1018       -4     
==========================================
- Hits          946      943       -3     
+ Misses         76       75       -1     

Flag	Coverage Δ
macos-latest	92.04% <100.00%> (+0.06%)	⬆️
py3.10	92.63% <100.00%> (+0.06%)	⬆️
py3.11	92.63% <100.00%> (+0.06%)	⬆️
py3.12	92.63% <100.00%> (+0.06%)	⬆️
py3.13	92.04% <100.00%> (+0.06%)	⬆️
py3.14	92.04% <100.00%> (+0.06%)	⬆️
ubuntu-latest	92.04% <100.00%> (+0.06%)	⬆️
windows-latest	92.63% <100.00%> (+0.06%)	⬆️

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