[Security Solution] Missing ECS fields in alerts (.alerts-security.alerts-default) index mappings

[vitaliidm]

Describe the bug:
Missing ECS fields in alerts index
It was reported in internal SDH ticket

Kibana/Elasticsearch Stack version:
At least 8.6+

Steps to reproduce:

1. Create a document with ECS field data_stream.namespace
2. Create rule that generates alert from those document
3. When an alert is generated, try to filter by data_stream.namespace from alert view
4. Field is not mapped in alerts view and filtering not working

Any additional context (logs, chat logs, magical formulas, etc.):

This field is not searchable, because data_stream.namespace is not in mapping of alerts index, even though its ECS field, because it is absent in a ECS fields map https://github.com/elastic/kibana/blob/8.7/x-pack/plugins/rule_registry/common/assets/field_maps/ecs_field_map.ts, that is used to provision .alerts-security.alerts-default index.

The solution could be to update this list, so it would contain all missing ECS fields.
But it raises further questions, how changes in ECS can be tracked and how to keep ECS fields up to date.

Similar ticket for non ECS fields in alerts view: #131623

[elasticmachine]

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

[vitaliidm]

issue is already covered in #129946