Remove 2.2 after 23rd of December 2019

[adamsitnik]

According to https://dotnet.microsoft.com/platform/support/policy/dotnet-core .NET Core 2.2 is going to reach the end of support at 23rd of December 2019

Since we have a LOT of CI legs and there is a lot of #if in the code I believe that it would be good to remove all 2.2 stuff after it becomes

• snap a branch with the existing state
• update .yml file and build matrix page
• update all #if NETCOREAPP2_2 usages
• update all docs that mention 2.2

[billwert]

I've added snapping a branch to the list - in the event we ever need to run 2.2 tests for some reason I'd like to have an easy way to do that.

[adamsitnik]

I've added snapping a branch to the list

good idea!