ci: bump actions/checkout to v6

[rejected-l]

Bumps to v6 for Node.js 24 support. Requires runner v2.329.0+.

https://github.com/actions/checkout/releases/tag/v6.0.0

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.36%. Comparing base (1822e84) to head (12f31e6).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1603   +/-   ##
=======================================
  Coverage   92.36%   92.36%           
=======================================
  Files          85       85           
  Lines        2278     2278           
=======================================
  Hits         2104     2104           
  Misses        174      174           

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