referencepolicy: finish backwards compatibility support and tests

[mikemorris]

Changes proposed in this PR:

• Simplify ReferencePolicy to ReferenceGrant conversion logic in gateway client
  • To match https://github.com/hashicorp/consul-api-gateway/blob/main/internal/k8s/controllers/http_route_controller.go#L141, possible since ReferencePolicy now uses ReferenceGrantSpec internally as the type of its Spec field.
• Fixup comment to properly refer to ReferenceGrant instead of ReferencePolicy
• Add tests for Gateway certificateRefs ReferenceGrant, and tests for ReferencePolicy backwards compatibility
• Add e2e tests for ReferencePolicy backwards compatibility

How I've tested this PR:

CI tests should still be passing after changes.

How I expect reviewers to test this PR:

Verify changes make sense and sufficiently cover support.

Checklist:

• Tests added
• CHANGELOG entry added

  Run make changelog-entry for guidance in authoring a changelog entry, and
  commit the resulting file, which should have a name matching your PR number.
  Entries should use imperative present tense (e.g. Add support for...)

[mikemorris]

e2e tests for Gateway CertificateRefs, HTTPRoute, and TCPRoute ReferencePolicy backwards compatibility fail as expected when this line is commented out, so the test coverage gap that existed when #280 was discovered is covered now.

[sarahalsmiller]

Will there be a point in time when referencepolicy is expected to fail, or will it keep working with the warning indefinitely?

[mikemorris]

Will there be a point in time when referencepolicy is expected to fail, or will it keep working with the warning indefinitely?

I believe the upstream plan is to remove ReferencePolicy in the next minor (v0.6.0) Gateway API release, so we would likely remove support when upgrading to that - could be several months out though.