Fix `RedisFailover` alert

From https://gitlab.com/gitlab-com/gl-infra/infrastructure/issues/6228:

During the last two incidents, we got alerts about Redis failover. This is a false-positive as no failover happened. The alert definition uses redis_connected_slaves, which changes for this incident, triggering the alert needlessly. Also its title can cause some confusion as it lists both types of redis (redis and redis-cache) which can sends us investigating the wrong way.