Remove "aggregation" from known events (for all events)

Problem

When defining a RedisHLL metric there are some keys that need to be defined in the known_events file, aggregation is one of them. To make the creation of metrics easier and have less differences between different types of metric we want to get rid of known_events

Proposal

There were talks about switching to weekly aggregation in general already here in this proposed spike: #361610 (closed)

Evaluate whether:

• aggregation setting provides value, and we need to migrate it so that user's can define it in the metric's *.yml file
• We have a more up-to-date method to achieve something similar as the provided setting and should migrate users to this
• The setting is not used at all or only rarely and we should deprecate this functionality completely.

Implementation Suggestion

With #390357 (closed) out of the way we should be able to just remove the setting and remove any documentation surrounding it.