Add API Diffs for .Net Core versions earlier than 3.0

[wtgodbe]

We have API diffs for all previews of .Net Core 3.0, starting with 3.0-preview1 vs. 2.2: https://github.com/dotnet/core/tree/master/release-notes/3.0/preview/api-diff. We should add diffs for versions earlier than 3.0, starting with 2.2 vs 2.1 and working backwards. Note that this is different from ApiCompat baselines, which exist in the Standard repo for compatibility of various frameworks (and old .Net Standard versions) with the current version of .NetStandard: https://github.com/dotnet/standard/tree/master/src/apicompat/baselines.

CC @karelz

[carlossanlop]

Hey @wtgodbe you self-assigned this. Have you done any progress on this issue?

[wtgodbe]

@Anipik did this for 2.2 vs 3.0 here: #3411

Will leave this open in case we want to continue backwards later on

[carlossanlop]

@Anipik did you get a chance to look into this?

[Anipik]

@carlossanlop i already added the api diff for 2.2 and 3.0. i think that we dont need to go back any further as 2.1 and 2.2 are going out of support.

[carlossanlop]

Okay, thanks @Anipik. Let's close this issue.