Add max_pinned_statuses to instances serializer and api response

[mjankowski]

Probably also needs doc update.

Resolves #25558

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.10%. Comparing base (86500e3) to head (30481a7).
Report is 241 commits behind head on main.

❗ Current head 30481a7 differs from pull request most recent head b061169. Consider uploading reports for the commit b061169 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #29441      +/-   ##
==========================================
+ Coverage   85.01%   85.10%   +0.08%     
==========================================
  Files        1059     1062       +3     
  Lines       28277    28352      +75     
  Branches     4538     4548      +10     
==========================================
+ Hits        24040    24129      +89     
+ Misses       3074     3061      -13     
+ Partials     1163     1162       -1     

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[renchap]

This will need a docs update once merged

[renchap]

@mjankowski can you open a docs PR with this change, and ping me on it for review?