worker/jobs/downloads/update_metadata: Remove crates.downloads column writes

[Turbo87]

This should remove a lot write operations from the crates table and hopefully improve the performance of the database a little bit.

If we see that this does not have the desired effect the following query should copy the values from the new crate_downloads table back into the crates table:

update crates
set downloads = (select downloads from crate_downloads where crate_downloads.crate_id = crates.id);

Related:

• Extract crate_downloads table #8232

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.81%. Comparing base (e57f029) to head (33db220).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8295      +/-   ##
==========================================
- Coverage   87.82%   87.81%   -0.01%     
==========================================
  Files         273      273              
  Lines       27522    27516       -6     
==========================================
- Hits        24170    24164       -6     
  Misses       3352     3352              

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