server,storage: enable separated intents

Jesse Seldess (jseldess) commented:

PR: https://github.com/cockroachdb/cockroach/pull/64831

From release notes:
> The default value of the `storage.transaction.separated_intents.enabled` [cluster setting](../v21.2/cluster-settings.html] was changed to `true`. [#64831][#64831)

Jira Issue: DOC-1374