This changes all of our uses of `NETCOREAPP3_1` to `NETCOREAPP`. There
is no value in distinguishing the exact .NET Core version we use in our
source tree as we only target one version. Also going forward it's not
expected that our .NET Core specific code will be version specific. If
that arises we can go back to version specific defines for those cases.

This also removes the last references to `netcoreapp1.1`,
`netcoreapp2.1` and `netcoreapp3.0` from our code.