Update edge docs for native sidecars enabled by default in Linkerd 2.20

[alpeb]

As of Linkerd 2.20 the proxy is injected by default as a native sidecar container (an init container with restartPolicy: Always) instead of a regular container. Update the edge docs accordingly:

• features/native-sidecars.md: state the new default and replace the "Enabling" section with instructions for disabling the mode, via the config.linkerd.io/proxy-enable-native-sidecar: false annotation at the namespace or workload level, or globally via the Helm value proxy.nativeSidecar: false
• features/proxy-injection.md, reference/architecture.md: mention the proxy is now injected as a native sidecar by default
• features/cni.md, tasks/grafana.md: scope the initContainer networking limitation (and its workarounds) to the case where native sidecars have been disabled, since init containers now have network access by default
• tasks/graceful-shutdown.md: meshed Jobs and Cronjobs now complete without extra configuration; scope the hanging-pod issue and the manual /shutdown workaround to the non-native-sidecar case. Drop the --wait-before-seconds option and the "Slow Updating Clients" section, which are no longer relevant with native sidecars
• tasks/adding-your-service.md: fix the mesh verification command to look for the proxy under spec.initContainers instead of spec.containers

[raykroeker]

I left a comment related to clarity around manual shutdown and a question.

[raykroeker]

Running multiple processes under the same UID feels like a general anti-pattern. I understand we recommend it elsewhere, should we continue to promote it?

[alpeb]

One advantage of this approach is that it keeps that specific init container out of the mesh, whereas the annotation alternative would apply to all containers in the pod. That said, I can see your point; it may be worth discussing further. If you feel strongly about it, please feel free to open a ticket so we can track it