Replace emojis with Unicode characters

[nxtn]

They don't render on https://octokitnet.readthedocs.io/en/latest/github-apps/

[codecov]

Codecov Report

Merging #2169 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2169      +/-   ##
==========================================
- Coverage   65.83%   65.81%   -0.03%     
==========================================
  Files         545      545              
  Lines       14234    14234              
  Branches      834      834              
==========================================
- Hits         9371     9368       -3     
- Misses       4706     4708       +2     
- Partials      157      158       +1     

Impacted Files	Coverage Δ
Octokit.Reactive/Helpers/ObservableExtensions.cs	90.29% <0.00%> (-2.92%)	⬇️

[shiftkey]

This is a great find, @NextTurn - thanks for submitting the fix!

[shiftkey]

release_notes: Updated GitHub Apps documentation to render emoji correctly