Release .NET 9.0.21 · dotnet/aspnetcore

What's Changed

[release/9.0] Update branding to 9.0.21 by @vseanreesermsft in #68962
[release/9.0] Update dependencies from dotnet/source-build-assets by @dotnet-maestro[bot] in #68967
[release/9.0] (deps): Bump src/submodules/googletest from 3940de9 to 36ba75f by @dependabot[bot] in #68919
[release/9.0] Update OpenApi to 1.6.31 by @Youssef1313 in #68837
[release/9.0] Extract IsAuthenticated helper method by @Youssef1313 with @Copilot in #68659
[release/9.0] Update dependencies from dotnet/arcade by @dotnet-maestro[bot] in #68570
[release/9.0] Update dependencies from dotnet/arcade by @dotnet-maestro[bot] in #69085
[release/9.0] Fix h3 connection-level and stream-level abort locking by @BrennanConroy in #69151
Merging internal commits for release/9.0 by @dotnet-bot in #69179
[release/9.0] [SignalR] [Java] Fix hang when reading an HTTP response body fails by @github-actions[bot] in #69208
[release/9.0] Fix: hardening http2 to refuse newline chars on trailers and dynamic HPACK table by @BrennanConroy in #69249
[release/9.0] Scope sections E2E assertions to mounted app by @wtgodbe in #69252
[release/9.0] Update dependencies from dotnet/arcade by @dotnet-maestro[bot] in #69268
Update MessagePack to 2.5.303 by @BrennanConroy in #69369

Full Changelog: v9.0.20...v9.0.21