Reenable ES support for public/internal project snippets

Background

ES support for public/internal project snippets was added in !18459 (merged), fixing #2358 (closed).

Since this required adding a join field to the ES index this caused #34558 (closed), so we reverted these changes in !19715 (merged).

Proposal

Revert the changes from !19715 (merged), but move them to a new ES mapping version.

This can be done once #10156 (closed) is in place, which will give instance administrators a supported way of upgrading ES index schemas.