Breaking: rename relaxed form of Atomic Operations extension

[bkoelman]

Breaking: Rename the relaxed form of Atomic Operations extension from "atomic-operations" to "atomic" for consistency. This exists for OpenAPI support in Content-Type and Accept HTTP headers, which is currently experimental.

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 90.69%. Comparing base (dc032d8) to head (7dfc2d0).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1664   +/-   ##
=======================================
  Coverage   90.69%   90.69%           
=======================================
  Files         354      354           
  Lines       11536    11536           
  Branches     1900     1900           
=======================================
  Hits        10462    10462           
  Misses        702      702           
  Partials      372      372           

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