[keyserver] Get rid of ExperimentalWarnings in keyserver logs

Summary: These clutter up logs which can be particularly annoying when you're logging stuff to debug (as I'm doing now).

Test Plan:
Before: There are a bunch of ExperimentalWarning messages
After: There are no ExperimentalWarning messages

Reviewers: ashoat, ginsu

Reviewed By: ginsu

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7204