Removing concept of resource-level support levels

[robscott]

What type of PR is this?
/kind cleanup
/kind documentation
/area conformance

What this PR does / why we need it:
The concept of conformance levels has gotten rather confusing as we've added both new features and resources. Some resources (GRPCRoute and ReferenceGrant) had conformance levels defined while the rest did not. This does some refactoring to try to make our documentation and lists of conformance features more consistent. This gets us to a point where:

1. Resources don't have support levels, implementations can choose to support whichever set of resources they want
2. Claiming the Support_Resource_ feature when running conformance tests means that you support the resource and all the core features it contains

As a result, the most notable change is that StandardCoreFeatures has been split into two: GatewayCoreFeatures and ReferenceGrantCoreFeatures, this more clearly matches the rest of our feature set. Hopefully this makes sense, but definitely let me know if this is just more confusing.

Does this PR introduce a user-facing change?:

Conformance Profiles: Some sets of features have been renamed to more closely match the resource they correspond to.

/hold
/cc @LiorLieberman @mlavacca @shaneutt

[shaneutt]

I think it makes sense to remove support levels at the resource level.

[arkodg]

/approve

[arkodg]

thanks for adding the comments
/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: arkodg, robscott, shaneutt

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[robscott]

/hold cancel