release-notes: document ClusterSettings/BackendSettings split as breaking

Addresses guydc's review comment on #8899 by documenting the Go-type
impact instead of maintaining both types side-by-side for a release.

Signed-off-by: Muhammad Waqar <mwaqar@confluent.io>

Author: muwaqar-cflt

release-notes/current/breaking_changes/8899-backend-cluster-settings-split.md

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+`ClusterSettings` in `api/v1alpha1` has been split: `ClusterSettings` now holds only the CDS-only fields (`loadBalancer`, `proxyProtocol`, `tcpKeepalive`, `healthCheck`, `circuitBreaker`, `timeout`, `connection`, `dns`, `http2`), and a new `BackendSettings` type embeds `ClusterSettings` and adds `retry`. YAML/JSON field paths are unchanged, but Go code that references these types directly is affected: `retry` moved off `ClusterSettings` onto `BackendSettings`, and `BackendCluster.BackendSettings` is now typed `*v1alpha1.BackendSettings` instead of `*v1alpha1.ClusterSettings`. Code that imports `api/v1alpha1` and constructs or type-references `ClusterSettings` directly (e.g. extension implementations) needs to update to the new types.