Add timed incremental rollout to Auto DevOps

What does this MR do?

This MR adds a new deployment strategy for AutoDevOps. The new strategy will allow users to continuously deploy to production with a timed-incremental rollout.

With this MR we introduce a new ~"auto devops" environment variable INCREMENTAL_ROLLOUT_MODE that can either be manual or timed.

We also deprecate the usage of INCREMENTAL_ROLLOUT_ENABLED, but we keep backward compatibility providing that value only for INCREMENTAL_ROLLOUT_MODE == "manual".

What are the relevant issue numbers?

Closes gitlab-org/gitlab-ee#7545

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides