Remove petab.v2-is-experimental warnings

[dweindl]

This has been communicated via the release notes. I don't think we need to annoy users with these warnings.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.20%. Comparing base (2ec51e8) to head (2e7677a).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #457   +/-   ##
=======================================
  Coverage   75.20%   75.20%           
=======================================
  Files          62       62           
  Lines        6855     6852    -3     
  Branches     1224     1224           
=======================================
- Hits         5155     5153    -2     
+ Misses       1233     1232    -1     
  Partials      467      467           

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