ci: use pinned image "mongo:5"

[unknwon]

Describe the pull request

Link to the issue: n/a

Checklist

• I agree to follow the Code of Conduct by submitting this pull request.
• I have read and acknowledge the Contributing guide.
• I have added test cases to cover the new code.

[codecov]

Codecov Report

Merging #57 (5caa073) into main (fba704d) will decrease coverage by 1.79%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main      #57      +/-   ##
==========================================
- Coverage   79.41%   77.61%   -1.80%     
==========================================
  Files           8        9       +1     
  Lines         544      621      +77     
==========================================
+ Hits          432      482      +50     
- Misses         63       78      +15     
- Partials       49       61      +12