Rename carts.json to openapi.json

[TharmiganK]

Purpose

Examples

Checklist

• Linked to an issue
• Updated the changelog
• Added tests
• Updated the spec
• Checked native-image compatibility

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 56.47%. Comparing base (1cbf6d4) to head (c9a3da1).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #11   +/-   ##
=======================================
  Coverage   56.47%   56.47%           
=======================================
  Files           2        2           
  Lines         255      255           
  Branches      179      179           
=======================================
  Hits          144      144           
  Misses        111      111           

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