Bump version to 2.60.0

[stanley-cheung]

For the final 2.60.0 release

[stanley-cheung]

Got an error in the "Nuget Build" pre-submit job with the following error:

MSBuild version 17.8.3+195e7f5a3 for .NET
  Determining projects to restore...
  Failed to download package 'Microsoft.CodeAnalysis.NetAnalyzers.8.0.0-preview.23472.1' from 'https://pkgs.dev.azure.com/dnceng/9ee6d478-d288-47f7-aacc-f6e6d082ae6d/_packaging/49a1bb2b-12b0-475f-adbd-1560fc76be38/nuget/v3/flat2/microsoft.codeanalysis.netanalyzers/8.0.0-preview.23472.1/microsoft.codeanalysis.netanalyzers.8.0.0-preview.23472.1.nupkg'.
  Response status code does not indicate success: 403 (Forbidden).

If I go to that URL directly, I got ERROR 403: Time-Limited URL validation failed.

Not sure if this is transient or there's something legit wrong...

[ Edit: confirmed the error was transient. Re-run the pre-submit job and they are fine now ]

[JamesNK]

It's a transient error in the package feed. Retry to see if it is fixed.