revert: roll back the broken 6.6.0 release

[jamescrosswell]

Part of the 6.6.0 release rewind. Refs #5260.

What this does

Reverts the release/6.6.0 merge (4a85500), which removes the 6.6.0 section from CHANGELOG.md. That's the only file that was modified by the original release commit — Directory.Build.props was never bumped, which is what caused the broken release in the first place (see #5261 for the underlying CI fix).

Why

The 6.6.0 release published GitHub assets named *.6.5.0.nupkg (root cause analysed in #5260 / #5261). NuGet rejected the upload as duplicates of the existing 6.5.0 packages, so nothing landed on nuget.org. We want to fully roll back and re-cut 6.6.0 cleanly once #5261 is merged.

Coordination

This PR is one of several steps. Recommended order:

1. Merge ci: restore version bumping via craft preReleaseCommand #5261 (.craft.yml preReleaseCommand fix).
2. Merge this PR.
3. Delete on GitHub: the 6.6.0 release, the 6.6.0 git tag, and the release/6.6.0 branch.
4. Re-run the Release workflow with version 6.6.0 (or auto).

#skip-changelog