Open 5.6.1 on release/5.x #200

Change

5.6.0 shipped from release/5.x at v5.6.0 (8975493) on 2026-10-23. Open 5.6.1 on it, as #177 opened 5.5.1:

• Version: Version 5.6.1, PackageValidationBaselineVersion 5.6.0.
• SECURITY.md: the supported-versions table names 5.6.x ✅ / < 5.6 ❌, the latest release on NuGet. SecurityPolicyConsistencyTests holds that row against the baseline.
• Changelogs: a ## [5.6.1] 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.

Done when

release/5.x prepares 5.6.1, so a fix PR into it adds its changelog line under 5.6.1.