.NET Core 2.1 has reached end of life

[adamsitnik]

dotnet/announcements#199

@billwert @Lxiamail are we interested in removing all usages of netcoreapp2.1 from the repo similarly to what we did for 2.2 when it got out of support (#1136)?

[Lxiamail]

@adamsitnik thanks for tracking the issue. That sounds the right thing to do. We will look into this.