Add support for OpenAPI 3.2

[martincostello]

Update to Microsoft.OpenApi 3.0.0 and add support for OpenAPI 3.2 document generation.

Depends on swagger-api/swagger-ui#10575 for swagger-ui and Redocly/redoc#2746 for Redoc, otherwise users won't be able to actually render OpenAPI 3.2 documents in the UIs we bundle.

TODO

• Add verify tests for OpenAPI 3.2 document
• Write migration guide
• Update README version table
• Manual testing
  • Test that the generated schemas are valid
  • Test that swagger-ui renders OpenAPI 3.2 documents (needs Support OpenAPI 3.2 swagger-api/swagger-ui#10575)
  • Test that Redoc renders OpenAPI 3.2 documents (needs Support OpenAPI 3.2 documents Redocly/redoc#2746)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.53%. Comparing base (d12336c) to head (8e1ce6b).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3646   +/-   ##
=======================================
  Coverage   94.53%   94.53%           
=======================================
  Files         111      111           
  Lines        3861     3861           
  Branches      778      778           
=======================================
  Hits         3650     3650           
  Misses        211      211           

Flag	Coverage Δ
Linux	94.53% <100.00%> (ø)
Windows	94.53% <100.00%> (ø)
macOS	94.53% <100.00%> (ø)

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