requirements: remove chardet < 6 pin, no longer needed with requests >= 2.32.6

[charmikadi]

Description

Remove the chardet < 6 pin from requirements.d/development.txt and
requirements.d/development.lock.txt.

This constraint was added as a workaround because requests <= 2.32.5
emitted RequestsDependencyWarning when chardet >= 6 was installed,
which caused noisy test output. Since requests >= 2.32.6 has now been
released with the upstream fix, this workaround pin is no longer needed.

Borg does not directly use chardet — it was only pinned to silence
warnings from requests. Removing it allows the package manager to
freely resolve chardet without the artificial constraint.

Fixes #9433

Checklist

• PR is against master (or maintenance branch if only applicable there)
• New code has tests and docs where appropriate
• Tests pass (run tox or the relevant test subset)
• Commit messages are clean and reference related issues

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.38%. Comparing base (cb83cc1) to head (18cb73c).
⚠️ Report is 12 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9567      +/-   ##
==========================================
- Coverage   83.40%   83.38%   -0.02%     
==========================================
  Files          87       87              
  Lines       15489    15489              
  Branches     2326     2326              
==========================================
- Hits        12918    12916       -2     
  Misses       1821     1821              
- Partials      750      752       +2     

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[ThomasWaldmann]

Thanks!