Deprecate and remove support for Redis 4

We've supported Redis 4 for a while, but since %12.7 we have shipped (via omnibus-gitlab: cf. omnibus-gitlab!3392 (merged)) and used Redis 5. We should first deprecate support for Redis 4:

%13.11

• Show warnings with rake gitlab:check that Redis 4 is outdated: !59072 (merged)
  • cf. !29395 (merged)

%13.12

• Add Redis 4 deprecation to the coming release post: gitlab-com/www-gitlab-com!79509 (merged)
  • cf. gitlab-com/www-gitlab-com!45801 (merged)

%14.0

• Backport gitlab-com/www-gitlab-com!79509 (merged) to 13.11 release blog: gitlab-com/www-gitlab-com!82489 (merged) by @dorrino
  • We did not make gitlab-com/www-gitlab-com!79509 (merged) in time for %13.11 so we needed to notify used 2 release post in advance for deprecations so we need to include Redis 4 in 13.11.
• Set the minimum version to 5 with rake gitlab:check that Redis 5 is not supported any more: !62156 (merged)
• Update software requirements docs: !62159 (merged)
• Update README.md of gitlab-org/gitlab: !62159 (merged)
  • cf. !39405 (merged)

%15.2

• docs: GitLab 13 requires Redis 5.0 or higher !90685 (merged)

Out of scope

• #366790 (closed)

Refs.

cf. #213174 (closed) (Deprecate and remove support for Redis 3)