Update support of gateway APIs in KIC docs for KIC 2.9

[randmonkey]

Where is the problem?

https://docs.konghq.com/kubernetes-ingress-controller/latest/references/gateway-api-support/

What happened?

KIC 2.9 will include the following updates on gateway APIs:

• bump gateway API version to 0.6.0
• promote ReferenceGrant to v1beta1, and enable it when Gateway feature gate turned on
• support GRPCRoute (alpha)

What did you expect to happen?

references/gateway-api-support page shows status of support of gateway API in KIC 2.9 after it is released.

Code of Conduct and Community Expectations

• [X] I agree to follow this project's Code of Conduct
• [X] I agree to abide by the Community Expectations