Multiplex connections between Praefect and Gitaly by default
This commit removes the gitaly_connection_multiplexing feature flag
and switches to multiplexed connections between Praefect and Gitaly.
The feature flag was introduced to performance test the connection in
production prior to fully switching them. No issues have come up, so
let's remove the feature flag and switch fully to multiplexed connections.
With the feature flag, only the routed RPCs would use the multiplexed connections. Now that they are the only connections available, the health checks and replication job operations will also use them.
Related to #3262 (closed)
Edited by Sami Hiltunen