chore: Write release notes for 0.28.0

[eseidel]

Update RELEASE_NOTES.md for 0.28.0

[eseidel]

This isn't ready for landing, since the new flutter engine isn't merged in, but I thought I'd get a head start on the release notes at least.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (2f2645b) to head (915ef52).

❗ Current head 915ef52 differs from pull request most recent head ec41755. Consider uploading reports for the commit ec41755 to get more accurate results

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #1827   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          173       173           
  Lines         5478      5478           
=========================================
  Hits          5478      5478           

Flag	Coverage Δ
shorebird_cli	100.00% <ø> (ø)

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