[keyserver] upgrade redis to version 7 on docker based keyserver

Summary:
Upgrades docker-based keyserver Redis to version 7

Depends on D12458

Test Plan: Launched docker-based keyserver and successfully performed basic user actions. Tested Redis by setting up a self-hosted keyserver and connecting to it with two different iOS simulators, sending messages live

Reviewers: bartek, varun, ashoat

Reviewed By: ashoat

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12475