Update releases documentation

[martincostello]

Update reference from AppVeyor to GitHub Actions.

[codecov]

Codecov Report

Merging #995 (4802ad3) into main (b99a563) will decrease coverage by 0.21%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #995      +/-   ##
==========================================
- Coverage   84.78%   84.57%   -0.22%     
==========================================
  Files         132      132              
  Lines        3228     3228              
==========================================
- Hits         2737     2730       -7     
- Misses        491      498       +7     

Flag	Coverage Δ
linux	84.57% <ø> (-0.22%)	⬇️
macos	52.75% <ø> (+52.75%)	⬆️
windows	52.78% <ø> (ø)

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Impacted Files	Coverage Δ
...ools/MessageHandling/Dispatch/MessageDispatcher.cs	85.45% <0.00%> (-12.73%)	⬇️

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