[macOS] Return the correct minor version on 11 and later OS. (#3605)

* [macOS] Return the correct minor version on 11 and later OS.

The OS returned from 11.0 and later is:

11.* = 20.6.0.0
12.* - 21.2.0.0

The minor in those versions is correct. This fix ensures that Agent
capabilities can match the minor version too.

Co-authored-by: Alexander Smolyakov <[email protected]>
Co-authored-by: Anatoly Bolshakov <[email protected]>

src/Microsoft.VisualStudio.Services.Agent/Capabilities/AgentCapabilitiesProvider.cs

@@ -117,7 +117,7 @@ private static string GetDarwinVersionString()
             }
             else
             {
-                return $"11.{version.Major - 20}";
+                return $"{version.Major - 9}.{(version.Minor > 0 ? version.Minor - 1 : version.Minor)}";
             }
         }
     }