Remove support for Linkerd + SMI

[stefanprodan]

Linkerd has deprecated the SMI integration and now it fully supports the Gateway API https://linkerd.io/2.18/features/gateway-api/

Tasks:

• Update Linkerd e2e to use Gateway API
• Remove support for SMI API and migrate the Linkerd specific code and unit tests to Gateway API
• Update the Linkerd tutorial to Gateway API
• Update the Flagger section in the Linkerd website

[sayap]

Hi Stefan, currently we rely on Flagger to manage SMI TrafficSplit resource, and then have our controller that reconciles on TrafficSplit and propagates the change. We don't use Linkerd, and also haven't started using the Gateway API yet.