Conformance supported-features flag help string should include all valid values.

[briantkennedy]

What would you like to be added:

https://github.com/kubernetes-sigs/gateway-api/blob/73f20f4ae4ab46a4a2ea52718fdbdef1a6726376/conformance/utils/flags/flags.go#L30C127-L30C127

The help string for -supported-features should include all valid feature values.

Why this is needed:

Users should not need to inspect source code to obtain the valid values.