Drop support for memcached

[rijkvanzanten]

Memcached doesn't support pub/sub, which means it can't be used for various things we need to rely on a central store for, including cross instance messaging, effective queuing, and websockets support.

Instead of trying to support memcached where it can be used, and use Redis for everything else, it makes more sense for the long term maintenance of the project to focus exclusively on Redis. Also ref #15176

[rijkvanzanten]

Linear: ENG-207