Expand batched background migration deletion docs
What does this MR do and why?
While working on cleaning up a batched background migration in !215101 (merged), I noticed that there are 2 endorsed ways of cleaning up BBMs across GitLab: manually deleting files, and relying on automatic migration squashing.
This MR expands the docs with:
- Acknowledgement that these are the 2 approved and safe strategies for achieving the goal of deleting BBM code.
- Step-by-step instructions for each strategy.
This will help reduce time spent in research and decision-making when developers want to clean up their BBM code.
References
- Came up in discussion on !215101 (merged) (see example MRs in description, and comments)
Screenshots or screen recordings
How to set up and validate locally
MR acceptance checklist
Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.
Related to #562154