.kibana_security_session_index_template causes deprecation log

[rudolf]

Related #120043

Starting Kibana with security enabled on this branch #120109 surfaces the following deprecation log:

ES DEPRECATION: 299 Elasticsearch-7.16.0-SNAPSHOT-34fb605e5a51f9ecbf4a6ad3d3b910402c5b2121 "Legacy index templates are deprecated in favor of composable templates."
 proc [kibana] Origin:user
 proc [kibana] Stack trace:
 proc [kibana]     at Indices.putTemplate (/opt/local-ssd/buildkite/builds/kb-n2-4-5be418573b67e921/elastic/kibana-pull-request/kibana-build-xpack/node_modules/@elastic/elasticsearch/lib/api/api/indices.js:976:16)
 proc [kibana]     at indexInitialization.Promise.finally.indexInitialization (/opt/local-ssd/buildkite/builds/kb-n2-4-5be418573b67e921/elastic/kibana-pull-request/kibana-build-xpack/x-pack/plugins/security/server/session_management/session_index.js:337:13)
 proc [kibana] Query:
 proc [kibana] 200
 proc [kibana] PUT /_template/.kibana_security_session_index_template_1
 proc [kibana] {"index_patterns":[".kibana_security_session_1"],"order":1000,"settings":{"index":{"number_of_shards":1,"number_of_replicas":0,"auto_expand_replicas":"0-1","priority":1000,"refresh_interval":"1s","hidden":true}},"mappings":{"dynamic":"strict","properties":{"usernameHash":{"type":"keyword"},"provider":{"properties":{"name":{"type":"keyword"},"type":{"type":"keyword"}}},"idleTimeoutExpiration":{"type":"date"},"lifespanExpiration":{"type":"date"},"accessAgreementAcknowledged":{"type":"boolean"},"content":{"type":"binary"}}}}

As per #120043 we should aim to fix the deprecation in 7.17 if this has a low risk

[elasticmachine]

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