Retire legacy DumpGCData SOS script

[max-charlamb]

Summary

Retire DumpGCData.script and its legacy registration after migrating the pinning-statistics regression to the modern SOS suite.

• Reuse the isolated DumpGCData debuggee with named unpinned and pinned stops and the existing snapshot helper.
• Assert zero pinning data before allocation and a value of one after pinning a GCHandle and forcing collection.
• Cover workstation GC live and both workstation/server GC in dumps through applicable native and dotnet-dump hosts. Preserve the existing Core-only scope; Framework and SingleFile are not added by this retirement.
• Explicitly skip .NET 8/9 in SOSTestSkips, with the original fix linked: [DAC] fix dac_gc_heap marshalling runtime#119393.
• Include the debuggee in the modern Helix payload catalog and update the coverage audit and README. No live-host GC wiring or CI pipeline changes.

Validation

Windows x64 local validation:

• Targeted SOS.Tests and SOS.UnitTests builds passed.
• 40 tests passed, including the five .NET 10 replacement configurations: CDB live workstation, and CDB/dotnet-dump dumps with workstation/server GC.
• After simplifying the matrix and adding explicit skips: 5 .NET 10 cases passed and 10 .NET 8/9 cases skipped.

Linux/macOS, x86/ARM, and .NET 11 were not run locally.