Update versioning docs to reflect recent changes in the release process.

[bexxmodd]

/kind documentation

What this PR does / why we need it: Update versioning docs to reflect recent changes in the release process.

Which issue(s) this PR fixes: Fixes #4282

Does this PR introduce a user-facing change?:

updating versioning docs located at https://gateway-api.sigs.k8s.io/concepts/versioning/

[k8s-ci-robot]

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[bexxmodd]

/cc @kflynn