chore: replaced kin-openapi library with libopenapi

[Prashansa-K]

deck file openapi2kong command uses kin-openapi for all its OpenAPI requirements. This library does not support OpenAPI 3.1, which is a requirement from the users. Thus, this change updates the library to libopenapi which can help us to adopt OpenAPI 3.1

For APIOps #1324 https://github.com/Kong/deck/issues/1324

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I didn't add much code refactoring in this PR for the aid of the review. I will take that up in a different PR.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 75.66667% with 73 lines in your changes missing coverage. Please review.

Project coverage is 68.19%. Comparing base (e464df7) to head (d14c3ed).

Files	Patch %	Lines
openapi2kong/openapi2kong.go	68.78%	27 Missing and 27 partials :warning:
openapi2kong/utils.go	82.75%	8 Missing and 2 partials :warning:
openapi2kong/jsonschema.go	78.12%	3 Missing and 4 partials :warning:
openapi2kong/validator.go	92.85%	1 Missing and 1 partial :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #194      +/-   ##
==========================================
+ Coverage   67.86%   68.19%   +0.33%     
==========================================
  Files          23       24       +1     
  Lines        2505     2550      +45     
==========================================
+ Hits         1700     1739      +39     
+ Misses        644      643       -1     
- Partials      161      168       +7     

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[Prashansa-K]

nice work @Prashansa-K !! Added some minor comments. Note that I don't know this go-apiops library so I think this work seems safe if the test suite and coverage are good.

Thank you! I will also test this out with some more examples to be thorough.