Replaces `composable index template` and `composable template` with
`index template` throughout data stream-related docs.
`Composable index template` is only used to contrast with legacy index
templates.
With #58096, data streams now track the timestamp field mapping outside
of the template associated with the stream. This means you can no longer
update the timestamp field mapping using template changes.
This updates the associated data stream docs.
James Rodewig