Use new EntityRepository

[colemanw]

This will be a long-term benefit to the stability of ECK: https://lab.civicrm.org/dev/core/-/issues/5410

The new EntityRepository, is intended to be flexible and comprehensive, with support for any storage method (sql-based, file-based, etc), and full support for virtual entities that are not tied to a DAO class.

I've already converted SearchKit's virtual entities over and it wasn't too hard. I'd like to do ECK sometime in the next 6 months or so, depending on BC compatibility goals (as it would necessitate a core version req bump).