Do not rely on index template for Kibana's user session index

[legrego]

The Elasticsearch Core Infra team is trying to fix an outstanding issue our users run into with system indices and templates. Essentially, Elasticsearch user index templates impact system indices in unexpected ways during upgrades, leading to unusable clusters. They are planning on preventing user defined templates to affect system indices in 8.4, which is what the original design intent of system indices was (but it was never implemented).

Our session index is currently configured using an index template. If we don't change anything, then
elastic/elasticsearch#87260 will break our session index, because it will no longer use the template we've configured.

We should:

• Move the index mapping and settings out of the template, and define them on the index itself.
• Delete the index template if it already exists in the cluster.

[elasticmachine]

Pinging @elastic/kibana-security (Team:Security)