[ML] Ensure annotations index mappings are up to date

[droberts195]

When the ML annotations index was first added, only the
ML UI wrote to it, so the code to create it was designed
with this in mind. Now the ML backend also creates
annotations, and those mappings can change between
versions.

In this change:

1. The code that runs on the master node to create the
   annotations index if it doesn't exist but another ML
   index does also now ensures the mappings are up-to-date.
   This is good enough for the ML UI's use of the
   annotations index, because the upgrade order rules say
   that the whole Elasticsearch cluster must be upgraded
   prior to Kibana, so the master node should be on the
   newer version before Kibana tries to write an
   annotation with the new fields.
2. We now also check whether the annotations index exists
   with the correct mappings before starting an autodetect
   process on a node. This is necessary because ML nodes
   can be upgraded before the master node, so could write
   an annotation with the new fields before the master node
   knows about the new fields.

Relates elastic/kibana#74935

[elasticmachine]

Pinging @elastic/ml-core (:ml)