Add a notice about OCI artifact name mismatch

Motivation

istioGateway is configured with an incorrect chartName (istio-gateway) in 2.54. This must be manually updated by the customer to gateway in order for the migration to work correctly on 2.54. A notice must be added to the migration guide documenting this workaround.

Acceptance Criteria

• A notice must be included in the 3.0 migration guide that explains the workaround above