Use `load-balancing-enabled=1` in `rserver.conf` when enabling Load Balancing

[cm421]

Description

Noticed this log in a customer's log that was using our helm chart
[rserver] WARNING Found /mnt/load-balancer/rstudio/load-balancer - load balancing enabled for compatibility with 2023.06. Add: load-balancing-enabled=1 or 0 to rserver.conf to explicitly enable load balancing to remove this warning.; LOGGED FROM: bool rstudio::server::load_balancer::isLoadBalanced() src/cpp/server/load_balancer/LoadBalancer.cpp:819

This is a warning and can be ignored, but to lower the noise and reduce the fallout that will happen if the legacy-supporting code in Workbench is removed we should include the load-balancing-enabled=1 flag in rserver.conf whenever replicas > 0.