Helm: Point flower extraNetworkPolicies deprecation at the real key (#72052)

The deprecation notice has named `flower.networkPolicy.peers` since the
parameter was restructured, but that key never shipped. `flower.networkPolicy`
is declared with `additionalProperties: false`, so anyone who follows the
notice hits a schema validation error and cannot install or upgrade at all.
The warning fires only when the deprecated parameter is set, so it reaches
exactly the users who are trying to migrate and sends them into a dead end.

Co-authored-by: Eason09053360 <185830721+Eason09053360@users.noreply.github.com>