Remove Redis 2/3 support from requirements doc

cf.
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/38475
https://gitlab.com/gitlab-org/gitlab/-/issues/213749
https://gitlab.com/gitlab-org/gitlab/-/issues/213174



Signed-off-by: Takuya Noguchi <takninnovationresearch@gmail.com>

README.md

@@ -81,7 +81,7 @@ GitLab is a Ruby on Rails application that runs on the following software:
 - Ubuntu/Debian/CentOS/RHEL/OpenSUSE
 - Ruby (MRI) 2.6.5
 - Git 2.8.4+
-- Redis 2.8+
+- Redis 4.0+
 - PostgreSQL 11+
 For more information please see the [architecture](https://docs.gitlab.com/ee/development/architecture.html) and [requirements](https://docs.gitlab.com/ee/install/requirements.html) documentation.