Consider a LTS release

We have a few customer+ that because of internal policy they can't update GitLab at the same pace we do. While I understand that in the past we dismissed the idea of having an LTS version as this could jeopardize the monthly release cycle and slow down the pace of how we release software, but MAYBE we could consider releasing it only for customers with Premium and/or Ultimate license.\

As this is something usually large companies would benefit the most, it makes sense to restrict it to the high paying tiers only.

So here is a simple proposal on how we could implement it:

• Every 4 months we release a new LTS release:
  • 22nd of January
  • 22nd of May
  • 22nd of September
• We support only the current LTS and the previous one
• Restrict package release and support only to users in Premium / Ultimate tiers. (different package server address with authentication per customer).

In a simple example, let's say the current release is promoted as LTS, that means that 11.3 would be supported with bugfixes and security fixes until 22nd of May 2019. So ideally when we release in 22nd of January, customers would start the compliance testing so they can upgrade to the 22nd January at any date before 22nd of May.

Read here for the motivation: https://gitlab.slack.com/archives/C32LCGC1H/p1538056902000100?thread_ts=1537997354.000100&cid=C32LCGC1H