Open 5.5.1 on release/5.x #177

Change

release/5.x was cut at v5.5.0 (490b4de) on 2026-10-20. Open 5.5.1 on it, as step 4 of the release and cut steps on #36 says:

• Version: Version 5.5.1, PackageValidationBaselineVersion 5.5.0.
• SECURITY.md: the supported-versions table names 5.5.x ✅ / < 5.5 ❌, the latest release on NuGet. SecurityPolicyConsistencyTests holds that row against the baseline.
• Changelogs: a ## [5.5.1] heading in all four. The root section stays empty and the package sections say "No change in this package." until a fix lands. The heading's date is set again in the commit that gets tagged (CONTRIBUTING.md, Changelog).

Done when

release/5.x prepares 5.5.1, so a fix PR into it adds its changelog line under 5.5.1.