[SIEM][Detections] Rule Actions SavedObject mapping uses dynamic: true

[rylnd]

As uncovered during our migration to the new SavedObject API #64029 (particularly this comment), SIEM currently uses dynamic: true for the mapping of a rule action's params property. This is discouraged in the new API for good reason, as it continues to create mappings as new fields are ingested.

As long as we're not using params for search/aggregation (needs verification), we should instead follow what e.g. alerting does for its params mapping:

{ "type" : "object", "enabled" : false }

[elasticmachine]

Pinging @elastic/siem (Team:SIEM)

[patrykkopycinski]

@rylnd Thank you for taking care of that. I didn't know that dynamic: true is not supported by NP and missed the approach taken by the Alerting team. Params are not used for search/aggregation, so feel free to change that to
{ "type" : "object", "enabled" : false }

[rylnd]

master/7.x: #64350
7.7.1: #64358