[Cases] Migrate Connector ID Fields to References

[jonathan-buttner]

This issue is to track the tasks for migrating the various connector ID fields to the references.

Tasks

Parent issue: #105677

• Migrate the configuration saved object's connector.id field to `references ([Cases] Migrate connector ID to references #104221)
• Migrate the cases saved object's connector.id and connector_id fields to references ([Cases] Migrate connector ID to references #104221)

Acceptance criteria

These tasks should not produce UI functionality difference in 7.15. So testing these changes really comes down to ensuring that existing functionality revolving around cases and the connectors for cases still works as it did in 7.14.

Here are the areas that will need to be tested:

• Upgrading from an older version to 7.15
  • Ensure that cases created in 7.14 with connectors (and which have been pushed to external services) render correctly in the UI with the appropriate connector icons and connector information after being upgraded to 7.15
  • Ensure that the user actions information is the same when viewing the 7.14 case with its connectors once the case is upgraded to 7.15
  • Ensure that selecting a connector, pushing a case, unselecting a connector all works as expected
• 7.15
  • Ensure that cases created in 7.15 with connectors that have/have not been pushed work and display correctly
  • Ensure that selecting a connector, pushing a case, unselecting a connector all works as expected

[elasticmachine]

Pinging @elastic/security-threat-hunting (Team:Threat Hunting)