A guide to planning a GitLab upgrade

What does this MR do?

This merge request creates some documentation to help customers plan for their GitLab upgrade.

Support currently helps customers with their upgrades and part of the requirements are that customers create a clear upgrade plan first. We've noticed that there isn't an easy way to tell if a plan is comprehensive and includes all necessary steps for each customer's specific environment. As a result, steps can be missed which can lead to problems including downtime and customer emergencies. This MR creates a guide to how to plan an upgrade, which includes creating a rollback plan and also accounts for various features (Geo, external Gitaly, etc).

The content of this MR was originally discussed in this MR and this issue.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team