Replace AppVeyor with GitHub Actions

[bkoelman]

AppVeyor has served us well for many years, but its development has stalled while the competition has caught up and improved far beyond what AppVeyor provides. And the Linux workers are magnitudes faster.

	AppVeyor	GitHub Actions
Build duration	55 min	20 min
Max parallel jobs	1	20 (macOS 5)
Job timeout	1 hour	6 hours (workflow 35 days)
Run subsequent steps after timeout	no	yes (enables to collect logs after hang)
Artifact/NuGet package retention	30 days/unlimited	90 days/unlimited
Build worker image updates	few times per year	weekly
Security	undocumented	very high
Timings per build step	no	yes
Test summaries	no	yes
Rich ecosystem of custom actions	no	yes
Run on .NET preview versions	manual	built-in
Recycle NuGet API key	requires commit	requires no commit
Suppress notification for canceled jobs	yes	no
Remote Desktop/SSH into build worker	yes	no

A bug fix in the coverage reporting causes the reported -3.8% below.

[codecov]

Codecov Report

Merging #1294 (6e30e3d) into master (f38a812) will decrease coverage by 3.80%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1294      +/-   ##
==========================================
- Coverage   96.82%   93.02%   -3.80%     
==========================================
  Files          21      268     +247     
  Lines         598     8777    +8179     
==========================================
+ Hits          579     8165    +7586     
- Misses         19      612     +593     

see 247 files with indirect coverage changes