Use codecov upload token

[AlekSi]

Description

Refs codecov/feedback#126.

Readiness checklist

• I added tests for new functionality or bugfixes.
• I ran task all, and it passed.
• I added/updated comments for both exported and unexported top-level declarations (functions, types, etc).
• I checked comments rendering with task godocs.
• I ensured that the title is good enough for the changelog.
• (for maintainers only) I set Reviewers (@FerretDB/core), Assignee, Labels, Project and project's Sprint fields.
• I marked all done items in this checklist.

[vercel]

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Name	Status	Preview	Updated
ferret-db	✅ Ready (Inspect)	Visit Preview	Oct 3, 2022 at 5:11PM (UTC)

[codecov]

Codecov Report

Merging #1204 (92717ef) into main (d35df61) will increase coverage by 0.04%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1204      +/-   ##
==========================================
+ Coverage   66.34%   66.39%   +0.04%     
==========================================
  Files         251      251              
  Lines       12299    12299              
==========================================
+ Hits         8160     8166       +6     
+ Misses       3247     3243       -4     
+ Partials      892      890       -2     

Impacted Files	Coverage Δ
internal/util/version/version.go	55.00% <0.00%> (-5.00%)	⬇️
internal/handlers/common/filter.go	87.52% <0.00%> (+0.94%)	⬆️

Flag	Coverage Δ
integration	62.66% <ø> (+0.04%)	⬆️
mongodb	14.84% <ø> (-0.03%)	⬇️
pg	51.90% <ø> (+0.04%)	⬆️
tigris	34.67% <ø> (-0.03%)	⬇️
unit	24.05% <ø> (-0.03%)	⬇️

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

[noisersup]

LGTM, one question :)