Reduce field count for SIEM saved object types

[rudolf]

SIEM is currently using 128 fields across the following types:

• siem-ui-timeline:96
• siem-ui-timeline-note:7
• siem-ui-timeline-pinned-event:6
• siem-detection-engine-rule-actions:9
• siem-detection-engine-rule-execution-info:10

Especially siem-ui-timeline with its 96 fields seems really high and from a rough search it seems like we might only be searching/aggregating over a handful of these fields. Could we reduce the field count of these types?

From https://github.com/elastic/kibana/blob/main/dev_docs/tutorials/saved_objects.mdx#mappings

Do not use field mappings like you would use data types for the columns of a SQL database. Instead, field mappings are analogous to a SQL index. Only specify field mappings for the fields you wish to search on or query. By specifying dynamic: false in any level of your mappings, Elasticsearch will accept and store any other fields even if they are not specified in your mappings.
Since Elasticsearch has a default limit of 1000 fields per index, plugins should carefully consider the fields they add to the mappings. Similarly, Saved Object types should never use dynamic: true as this can cause an arbitrary amount of fields to be added to the .kibana index.

Related #43673

[elasticmachine]

Pinging @elastic/siem (Team:SIEM)