Update docs to reference gateway api v1

[ThatsMrTalbot]

Cert Manager was updated to use v1 of the GatewayAPI in this PR https://github.com/cert-manager/cert-manager/pull/6559

This is the matching documentation update

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: inteon

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[inteon]

@ThatsMrTalbot small mistake on my end here, this should have been merged into the release-next branch.

[ThatsMrTalbot]

Ah, so revert and re-raise?

[inteon]

I'm afraid so.

[ThatsMrTalbot]

Raised https://github.com/cert-manager/website/pull/1370 to revert