Minor documentation fixes

The System Requirements in /Docs/reference/content/getting_started/installation.md state that the NuGet packages have 2 target frameworks, net45 and netstandard1.5. This should be updated to note that they have 4 target frameworks, net452, netstandard1.5, netstandard2.0, and netstandard2.1.

/Docs/reference/content/upgrading.md mentions no known backward compatibility issues with 2.7.0-2.10.x. This should be 2.11.x.