WIP: Make upgrading to 12.0 mandatory to upgrade to other 12.x versions

In 12.0 release post we mention

When upgrading to a future version such as 12.1.0, users must first upgrade to 12.0.0. Failing to do so may result in migrations not being applied, which could lead to application errors.

This was not what we were following till now. What we made mandatory was users should be on last minor version of 11.x before upgrading to any version in 12.x (Check the example in https://docs.gitlab.com/ee/policy/maintenance.html#upgrade-recommendations). This was essentially for two reasons

1. To prevent users jumping from 9.x to 12.x in one go, thus breaking multiple things.
2. To ensure users are aware of all the deprecation warning we put in during 11.x releases - last release of 11.x will have all of them.

This new change means for someone on 11.2 to upgrade to 12.3, the jumps are

1. 11.2 => 11.11
2. 11.11 => 12.0
3. 12.0 => 12.3