Fix the switchover_patroni_leader ansible runbook to not update the leader with tags

In a recent run, we noticed that the runbook https://gitlab.com/gitlab-com/gl-infra/db-migration/-/blob/master/dbre-toolkit/switchover_patroni_leader.yml?ref_type=heads is removing the following tags placed on the existing leader.

nofailover: true
noloadbalance: true

We would like to modify the runbook so that it does not modify any existing tags on a leader node during failover.

cc @rhenchen.gitlab