gossip: convert all connections to validated

Gossip is one of the few places in the code that uses `GRPCUnvalidatedDial`. This is a requirement for bootstrapping the system, however it results in connections that have two problems:
1) They don't correctly disconnect if there is an asymmetric partition
2) They don't correctly pass in the locality for the remote node.

The first can result in strange gossip partitions because the gossip network assumes connections are symmetric. The second results in uncategorized network traffic.

A potential fix would be to use the unvalidated dial for bootstrapping, but once the node has joined the gossip network, it would drop the original connection and reconnect with a validated dial.

Jira issue: CRDB-42253