Allow installing Redis 7 from the Redis role

[acozine]

Currently the ubuntu repos only support up to Redis 6.

To upgrade to Redis 7 manually, you can do:

sudo add-apt-repository ppa:redislabs/redis
sudo apt-get update
sudo apt upgrade redis

Make this possible with the redis role, so that we would not install the wrong version of redis if we rebuilt bibdata or other redis-dependent applications.

Note: when doing this manually, a warning pops up that using the PPA is not preferred. We may need a way to acknowledge the warning in the role.