The future of MergeGateways mode

[zirain]

When we were discussing #8623 at the community meeting, there's one thing that we need to figure out. What is the future of MergeGateays mode as ListenerSet promoted to v1 in Gateway API v1.5, and will be supported in the coming Envoy Gateway v1.8.
MergeGateways was introuduced when there's no ListenerSet on upstream, but today it's time to revist it again.

cc @cnvergence who mainly contribute on this area.

cc @envoyproxy/gateway-maintainers for more insight

[arkodg]

cc @guydc

[github-actions]

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