switch to Microsoft.SourceLink.GitHub package to perform linking

[shiftkey]

I want to do away with the use of DotNetCliToolReference inside here (there's a kludge that I'll explain when we get to it) and for now it looks like Microsoft.SourceLink.* are the current ways to achieve this (with some options that I might look into later).

For now, I just want to confirm this works and passes sourcelink test for each build.

[codecov]

Codecov Report

Merging #2074 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2074   +/-   ##
=======================================
  Coverage   67.06%   67.06%           
=======================================
  Files         538      538           
  Lines       14138    14138           
=======================================
  Hits         9481     9481           
  Misses       4657     4657

[shiftkey]

release_notes: Switch over to Microsoft.SourceLink.GitHub for embeddeding GitHub URLs into assemblies