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