Document new group_replication options in 8.0.30

PS 8.0.30 introduces two new GR options, PS-8275 and PS-8276.

PS-8275 adds a new system variable, group_replication_auto_evict_timeout. The default value of the variable is 0, which doesn't change existing behavior. If it is non-zero, it changes the flow control logic: if a node exceeds the flow control threshold for more seconds than what the variable specifies, it will automatically leave the cluster. The only exception from this is the primary node in single primary mode.

PS-8276 adds a new value to the group_replication_flow_control_mode system variable, "MAJORITY". In this new mode, flow control only gets activated if the majority (more than half) of the nodes exceed the flow control threshold. Existing (upstream) values of the variable continue to work as before.