refactor(backend): Use promises API

[woxtu]

What

Promises APIがあるメソッドはそれを使うように変更します。

Why

都度util.promisify()しなくて良くなると嬉しい。

Additional info (optional)

Checklist

• Read the contribution guide
• Test working in a local environment
• (If needed) Add story of storybook
• (If needed) Update CHANGELOG.md
• (If possible) Add tests

[codecov]

Codecov Report

Merging #11351 (5c2118f) into develop (b546d41) will decrease coverage by 0.02%.
The diff coverage is 87.50%.

@@             Coverage Diff             @@
##           develop   #11351      +/-   ##
===========================================
- Coverage    77.69%   77.68%   -0.02%     
===========================================
  Files          918      919       +1     
  Lines        92621    92626       +5     
  Branches      7748     7748              
===========================================
- Hits         71966    71955      -11     
- Misses       20655    20671      +16     

Impacted Files	Coverage Δ
packages/backend/src/core/DownloadService.ts	70.00% <66.66%> (-0.40%)	⬇️
packages/backend/src/core/FileInfoService.ts	67.60% <100.00%> (-0.31%)	⬇️
packages/backend/src/server/api/ApiCallService.ts	74.87% <100.00%> (-0.19%)	⬇️

... and 7 files with indirect coverage changes

[saschanaz]

いいと思います👍

[syuilo]

👍🏻