[Build][CI] Delete a release by its git reference

[Nice3point]

Deletes the NuGet packages of a release by the versions its tag records.

What changed

• The Delete release workflow runs on dispatch and takes the release tag as the ref input.
• The deletion command of the build is delete, in place of clean-nuget.
• The deleted versions come from Build:Versions of build/appsettings.json at the given reference, read through git show without a checkout.
• Delete:Ref holds the reference, and the workflow sets it from the input.

Where to look

• A tag older than the ModularPipelines build has no build/appsettings.json. The run fails on git show and deletes nothing.

How verified

• dotnet build -c Release of the build project: zero errors.
• git show 2027.0.0-preview.5.20260329:build/appsettings.json returns the versions of that release.
• Not covered: a run against nuget.org.