Implement Composable Templates to support Elasticsearch 8.0

[yaauie]

Elasticsearch's /_template API is being deprecated in 7.8, and is slated for removal in 8.0, so we need to migrate to Composable Templates (see: elastic/elasticsearch#53101).

We can begin our implementation as soon as the newly-introduced Composable Templates API is marked Stable (slated for 7.9), and should use it whenever we are communicating with an Elasticsearch 8 cluster.