Update core C2PA SDK to v0.90.21

[github-actions]

Bumps c2paVersion to v0.90.21.

Opened by the track/c2pa-rs-stable tracker after make library, make tests-with-server
and the full emulator matrix all passed against this version.

Left as a PR rather than auto-merged: a bump that compiles and tests clean can
still change signing behaviour.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.21%. Comparing base (dcfd49a) to head (33e7ac7).

Additional details and impacted files

@@            Coverage Diff            @@
##               main     #137   +/-   ##
=========================================
  Coverage     72.21%   72.21%           
  Complexity       38       38           
=========================================
  Files            62       62           
  Lines          2350     2350           
  Branches        286      286           
=========================================
  Hits           1697     1697           
  Misses          495      495           
  Partials        158      158           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.