[ML] Make sure new fields .ml-annotations* work correctly

[przemekwitek]

Recently, a number of fields in .ml-annotations* have been added:

• detector_index, partition_field_name, partition_field_value, over_field_name, over_field_value, by_field_name, by_field_value in Add scope-related fields to Annotation elasticsearch#56417
• event in Introduce Annotation.event field elasticsearch#57144 and [ML] Add a new annotation type for categorization status changes elasticsearch#58394

They are/will be available from version 7.9.
Please make sure this backend change is BWC wrt Kibana code.

[elasticmachine]

Pinging @elastic/ml-ui (:ml)

[peteharverson]

Closing as these new fields are now in use with the enhancements that were added for annotations for 7.9.0 in #70198.