Migrate from istio inject annotation to label

[ReToCode]

Context

We use the istio annotation sidecar.istio.io/inject: true in Client and related extension repos. Istio deprecated the annotation a while ago (ref: https://istio.io/latest/docs/reference/config/analysis/ist0135/ and https://istio.io/latest/docs/reference/config/annotations/#SidecarInject). Downstream this starts to cause issues (e.g. with OSSM 3.x), so we should migrate to the according label.

cc @dsimansk this came up here. I was unable to get the "old" way to work with OSSM 3.x, but we probably should migrate to the label anyway (we updated the docs for the customer to do the same).

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