add upload_token_required_for_public_repos, backfill and make non-nullable

[nora-codecov]

codecov/engineering-team#2297

[Swatinem]

lgtm.
So this will create the column as NULL, the backfill sets that to true, and then later on the NULL is removed, and the default false remains?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.11%. Comparing base (a97eb0a) to head (fc0b7f0).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #373      +/-   ##
==========================================
- Coverage   89.74%   89.11%   -0.64%     
==========================================
  Files         380      324      -56     
  Lines       11736     9694    -2042     
  Branches     2071     1737     -334     
==========================================
- Hits        10533     8639    -1894     
+ Misses       1114      991     -123     
+ Partials       89       64      -25     

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