Return accurate results for Activities API latest endpoint

[activefx]

This enables the Activity API's latest endpoint to return the 50 most recently created gems. Previously, the endpoint used the new_pushed_versions scope which returns new gems with only one release, preventing the endpoint from returning new gems that are updated recently after their release.

Resolves #5741

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.39%. Comparing base (3a1b246) to head (00b2333).
Report is 6 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5778      +/-   ##
==========================================
- Coverage   97.15%   94.39%   -2.77%     
==========================================
  Files         460      460              
  Lines        9568     9627      +59     
==========================================
- Hits         9296     9087     -209     
- Misses        272      540     +268     

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