Upgrade Redis to 6
Further info: https://github.com/discourse/discourse_docker/commit/86b9032bbe75cfd11476c44a5c62f8882dd8ad97
But in the Gemlock, redis is still 4.0.
Further info: https://github.com/discourse/discourse_docker/commit/86b9032bbe75cfd11476c44a5c62f8882dd8ad97
But in the Gemlock, redis is still 4.0.