Update validateBackendRef invalid group error conditions

[ciarams87]

Proposed changes

Problem: The HTTPRouteInvalidBackendRefUnknownKind conformance test expects ResolvedRefs/False/InvalidKind condition to be set on the HTTPRoute if a BackendRef specifies an unknown kind.

Solution: We actually do set the correct condition in this case where just Kind is invalid but Group is valid, but when Group is invalid, we set a different condition. Solution is to update the condition reason to InvalidKind when either Kind or Group is invalid. Comment for context from @pleshakov:

because Kind depends on the group (for example, the fact that the kind is Service (valid) is meaningless if the group is not core), I think the current logic is correct. Perhaps we can just report conditions.NewRouteBackendRefInvalidKind for the group error as well, without changing the order, since the group is kind of related to the kind anyway and there is nothing in the spec written about the group validation.

Testing: The failing conformance test now passes successfully:

    --- PASS: TestConformance/HTTPRouteInvalidBackendRefUnknownKind (3.04s)
        --- PASS: TestConformance/HTTPRouteInvalidBackendRefUnknownKind/HTTPRoute_with_Invalid_Kind_has_a_ResolvedRefs_Condition_with_status_False_and_Reason_InvalidKind (0.01s)
        --- PASS: TestConformance/HTTPRouteInvalidBackendRefUnknownKind/HTTP_Request_to_invalid_backend_with_invalid_Kind_receives_a_500 (0.00s)

Closes #776

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• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
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[sjberman]

@ciarams87 Would you be able to update the commit/PR description after your changes? I was a bit confused as to what was happening