[release-1.5] CORS: clarify API documentation and remove conformance tests for credentialed requests

[snorwin]

What type of PR is this?

/kind cleanup
/kind documentation

What this PR does / why we need it:
Manual backport of #4663.

Which issue(s) this PR fixes:

N/A

Does this PR introduce a user-facing change?:

Clarified CORS API documentation to describe behavior based on `allowCredentials` instead of credentialed requests.

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[youngnick]

Since v1.5.1 has been released, this would imply we would need a v1.5.2?

[snorwin]

@youngnick since this is purely a documentation change, I’m not sure it justifies a 1.5.2 release. I’ve backported it so that if we do ship another patch release, it will be included.