SDK Diff Pipeline not triggering for 8.0 and 9.0

[ellahathaway]

The source-build official pipeline was recently renamed from dotnet-source-build to dotnet-source-build-pre10.0. As a result, the SDK diff pipeline is no longer triggered for the 8.0 and 9.0 branches, since they still reference the old dotnet-source-build pipeline name.

[MichaelSimons]

@ellahathaway - Can this be closed now that the two linked PRs have been merged? TIA