add published_date field to experiment schema

[jeddai]

O(n) change

Pull Request checklist

• Breaking changes: This PR follows our breaking change policy
  • This PR follows the breaking change policy:
    • This PR has no breaking API changes, or
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Branch builds: add [firefox-android: branch-name] to the PR title.

[eliserichards]

👏 🚀

[jhugman]

r+wc. Nice!

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (aa5572f) 26.09% compared to head (862a008) 26.18%.
Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5936      +/-   ##
==========================================
+ Coverage   26.09%   26.18%   +0.09%     
==========================================
  Files         375      375              
  Lines       47292    47335      +43     
==========================================
+ Hits        12339    12397      +58     
+ Misses      34953    34938      -15     

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