Add Deprecation Notices for Envoy-based AI Gateway and Inference Extension

[lgadban]

Per community decision and shift in focus from Envoy to agentgateway for AI features, the various envoy-based AI features should be marked as deprecated for the 2.1.x release (to be removed in 2.2.x).

[danehans]

xref: #10878 for deprecating the Inference EPP deployer.

[Nadine2016]

deprecation note added for docs in kgateway-dev/kgateway.dev#416