Support OpenAPI v3.1

[zijianhuang]

after Microsoft's parser component is upgraded for v3.1 @ microsoft/OpenAPI.NET#795

And construct a test suite only for v3.x, those openapi.yaml (1965 as of 2024-01-14) in OpenAPI Directory.

Many are on v3.1.

[zijianhuang]

https://github.com/microsoft/OpenAPI.NET/blob/main/docs/upgrade-guide-2.md
but must be with .NET 10.
Microsoft .NET 10 is projected for a release in November 2025.