chore: Add support for Python 3.14 and remove support for 3.11

[john-halloran]

Process consisted of:

• Reading the Python 3.12-3.14 release notes.
• Confirming that nothing we use was deprecated/changed in 3.14 or reliant on 3.11.
• Asking AI to double check this; it agreed.
• Updating pyproject.toml and cookiecutter.json with the new versions.

Closes #172.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.32%. Comparing base (d7586bc) to head (5af6af3).
⚠️ Report is 15 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #211      +/-   ##
==========================================
+ Coverage   77.50%   87.32%   +9.82%     
==========================================
  Files           3        3              
  Lines          40       71      +31     
==========================================
+ Hits           31       62      +31     
  Misses          9        9              

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