fix(MeshGatewayInstance): don't overwrite annotations/labels in managed Service (backport of #7069)

[kumahq]

Automatic cherry-pick of #7069 for branch release-1.8

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cherry-picked commit 07cf4ca
⚠️ ⚠️ ⚠️ Conflicts happened when cherry-picking! ⚠️ ⚠️ ⚠️

On branch release-1.8
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	both modified:   pkg/plugins/runtime/k8s/controllers/gateway_instance_controller.go

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