Bump recommended Redis version from 4.0 to 5.0

What does this MR do?

Bumps recommended Redis version from 4.0 to 5.0 to announce the removal of Redis 4.x (< 5.0) support in %14.0 to GitLab instance administrators through rake gitlab:check command.

Screenshots (strongly suggested)

With Redis 4.0.9 (=> warning)

$ rake gitlab:check
[...]
Administrator / Long Long Long Long.... ... yes
Redis version >= 5.0.0? ... no
  Try fixing it:
  Support for your Redis version 4.0.9 has been deprecated and will be removed soon. Update your Redis server to a version >= 5.0.0
  For more information see:
  doc/administration/redis/index.html#redis-replication-and-failover-using-the-non-bundled-redis
  Please fix the error above and rerun the checks.
Ruby version >= 2.7.2 ? ... yes (2.7.2)
[...]

With Redis 5.0.0 (=> no warning)

$ rake gitlab:check
[...]
Administrator / Long Long Long Long.... ... yes
Redis version >= 5.0.0? ... yes
Ruby version >= 2.7.2 ? ... yes (2.7.2)
[...]

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
• Code review guidelines
• Style guides
• Separation of EE specific content

Related to #327197 (closed)