Versioning CRD yaml to simplify only installing v1alpha2

[robscott]

What type of PR is this?
/kind cleanup

What this PR does / why we need it:
This moves CRD yaml to version specific directories that will simplify installing only v1alpha2 CRDs. This also adds the ReferencePolicy CRD to kustomize config. Although I'd initially tried to remove the v1alpha1 CRDs entirely, we rely on them to verify our v1alpha1 examples. Until we release v1alpha2, I think we need to leave that verification, and therefore the v1alpha1 CRDs in place, even though we should be encouraging only installing v1alpha2 CRDs post-release.

Some reasons for this:

• Since the v1alpha1 and v1alpha2 CRDs exist in different API groups, having both in a cluster will be confusing when using kubectl. (kubectl get httproute may return v1alpha1 or v1alpha2 resources, but never both). Users will also not see any warning about the duplicate resource names in their cluster.
• The existing v1alpha1 CRDs are frozen and can still be installed by following the documentation: https://gateway-api.sigs.k8s.io/v1alpha1/guides/getting-started/#installing-gateway-api-crds-manually.
• Many/most implementations plan to move directly to only supporting v1alpha2 CRDs.

Of course we could take this a step further and remove the entirety of the v1alpha1 APIs and related codegen, but I think it's helpful to leave both in place if any implementations do want to concurrently support both versions.

Note: This also removes hack/verify-crds.sh. I spent some time trying to make it work with this new structure before I realized hack/verify-codegen.sh already completely covers this in a simpler way.

Does this PR introduce a user-facing change?:

v1alpha1 CRD yaml has been removed in preparation for v1alpha2 release. These CRDs can still be installed from v0.3.0.

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: howardjohn, robscott

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[youngnick]

/lgtm