Using "Set" list type for "Remove" in Header Modifier Filters

[robscott]

What type of PR is this?
/kind cleanup

What this PR does / why we need it:
This fills one more gap in our CRD validation that we've historically relied on the validating webhook to cover. As documented in #2277, the webhook ensures that header names have case-insensitive uniqueness. The built-in CRD validation uses +listType=map and +listMapKey=name to ensure that names are unique (unfortunately still case-sensitive, but better than nothing). We were missing equivalent CRD validation on the Remove field, this adds that.

As we're moving away from webhook-based validation, adding as much validation we can directly to the CRD will be helpful, so this takes one more step in that direction.

Does this PR introduce a user-facing change?:

HTTPRoute and GRPCRoute CRDs now provide built-in validation that ensures the uniqueness of names in Header Modifier "Remove" lists.

[youngnick]

/lgtm

[k8s-ci-robot]

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