[Security Solution][Detections] Change in `set-value` behavior affecting Security ML Detection Rule

[spong]

In 7.15 the set-value dependency was updated to 4.1.0 (from 3.0.2), which introduced a huge behavioral change when numbers are provided as the 2nd parameter. When a field name is something like 123.788 this will no longer generate an object: { 123: { 788: { ... } } but instead { 123: [ , , , , /* 788 empty elements */, { ... } ] }.

Note: This should be an unlikely error-scenario as there are presumably few occurrences where field.name's are all numbers within this context (citation needed), but in any case this will either result in trying to write malformed data to alerts or the rule failing execution outright.

Usage as follows: https://github.com/elastic/kibana/blob/4681a80317176543535bfdb2188d94aa40d741fc/x-pack/plugins/security_solution/server/lib/detection_engine/signals/bulk_create_ml_signals.ts#L57-L63

[elasticmachine]

Pinging @elastic/security-solution (Team: SecuritySolution)

[elasticmachine]

Pinging @elastic/security-detections-response (Team:Detections and Resp)