Shouldn't only leader node remove outdated docs from nodes collection?

## Expected Behavior
Only leader node remove outdated docs from nodes collection.

## Current Behavior
All nodes do it in AbstractNodeService#ping() method every second.
It means that in 16-nodes-big cluster each second Mongo is asked 16 times to remove stale node information.
It seems to be unnecessary.
It may potentially create funny issues when client has `stale_leader_timeout` set to different values in different nodes.

## Possible Solution
Only leader calls dropOutdated() method.

## Context
May be loosely related to HS-2290103299.

## Your Environment
* Graylog Version: 5.2.3
* MongoDB Version: 5