Pin posthog to <6.0.0

[Davidyz]

See chroma-core/chroma#4966

Since we're still using chromadb 0.6.3, we'd have to do it ourselves.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.49%. Comparing base (a577112) to head (a90e902).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #222   +/-   ##
=======================================
  Coverage   99.49%   99.49%           
=======================================
  Files          21       21           
  Lines        1589     1589           
=======================================
  Hits         1581     1581           
  Misses          8        8           

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