Don't use fs.promises to prevent warnings

[AndyBitz]

Don't use fs.promises to prevent warnings from node.

[codecov-io]

Codecov Report

Merging #2602 into canary will decrease coverage by <.01%.
The diff coverage is 55.55%.

@@            Coverage Diff            @@
##           canary   #2602      +/-   ##
=========================================
- Coverage    7.95%   7.94%   -0.01%     
=========================================
  Files         258     258              
  Lines        9401    9397       -4     
  Branches     1068    1067       -1     
=========================================
- Hits          748     747       -1     
+ Misses       8604    8601       -3     
  Partials       49      49

Impacted Files	Coverage Δ
src/util/read-json-file.ts	0% <ø> (ø)	⬆️
src/commands/deploy/legacy.js	0% <ø> (ø)	⬆️
src/commands/deploy/index.js	0% <ø> (ø)	⬆️
src/util/git.js	0% <ø> (ø)	⬆️
src/util/index.js	0% <0%> (ø)	⬆️
src/util/dev/server.ts	11.28% <0%> (ø)	⬆️
src/util/hash.js	100% <100%> (ø)	⬆️
src/util/read-metadata.js	100% <100%> (ø)	⬆️
src/util/get-files.ts	86.5% <75%> (ø)	⬆️
src/util/metrics.ts	68.18% <0%> (+6.64%)	⬆️

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