Use node v15 in CI

[JoeRobich]

No description provided.

[codecov]

Codecov Report

Merging #4480 (fa6eac4) into master (21fd01b) will increase coverage by 0.32%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #4480      +/-   ##
==========================================
+ Coverage   68.76%   69.08%   +0.32%     
==========================================
  Files         108      108              
  Lines        4306     4273      -33     
  Branches      747      739       -8     
==========================================
- Hits         2961     2952       -9     
+ Misses       1139     1121      -18     
+ Partials      206      200       -6     

Flag	Coverage Δ
integration	52.72% <100.00%> (-0.83%)	⬇️
unit	85.81% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/features/codeActionProvider.ts	70.96% <100.00%> (ø)
src/statusBarItemAdapter.ts	50.00% <0.00%> (-3.34%)	⬇️
src/omnisharp/delayTracker.ts	58.33% <0.00%> (-3.21%)	⬇️
src/coreclr-debug/util.ts	48.64% <0.00%> (-1.36%)	⬇️
src/features/codeLensProvider.ts	54.54% <0.00%> (-1.27%)	⬇️
src/features/renameProvider.ts	82.35% <0.00%> (-0.99%)	⬇️
src/features/documentSymbolProvider.ts	95.65% <0.00%> (-0.35%)	⬇️
src/omnisharp/protocol.ts	87.41% <0.00%> (-0.09%)	⬇️
src/coreclr-debug/activate.ts	35.00% <0.00%> (ø)
src/features/json/projectJSONContribution.ts	7.95% <0.00%> (+0.08%)	⬆️
... and 8 more

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