.net 9 migration

[twcclegg]

Changes

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.27%. Comparing base (8a39ff6) to head (ac35768).
Report is 5 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #273   +/-   ##
=======================================
  Coverage   75.27%   75.27%           
=======================================
  Files          34       34           
  Lines        4424     4424           
  Branches     1001     1001           
=======================================
  Hits         3330     3330           
  Misses        885      885           
  Partials      209      209           

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[wmundev]

does this need updating too? https://github.com/twcclegg/libphonenumber-csharp/blob/main/lib/github-actions-metadata-update.sh#L116

otherwise looks good to me, nice one!