Remove Experimental attribute from AddOpenApiForJsonApi

[bkoelman]

Remove [Experimental] from AddOpenApiForJsonApi, now that preview packages are deployed on NuGet.
The message is redundant because the version on NuGet already indicates "preview" (and it is annoying).

QUALITY CHECKLIST

• Changes implemented in code
• Complies with our contributing guidelines
• Adapted tests
• N/A: Documentation updated

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.78%. Comparing base (7a59c74) to head (f546356).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1724   +/-   ##
=======================================
  Coverage   90.78%   90.78%           
=======================================
  Files         467      467           
  Lines       14615    14615           
  Branches     2302     2302           
=======================================
  Hits        13268    13268           
  Misses        917      917           
  Partials      430      430           

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