need to set prefer-chassis-as-gw=true explicitly in caracal

[dosaboy]

ovn changed in 24.03 (caracal) and defaults to not schedule any gateways if this is not set and as it was never set by default in the charm we will need to set it if we want gateways to be scheduled.

[1] https://charmhub.io/ovn-chassis/configuration#prefer-chassis-as-gw