Package gitaly fleet work for next upgrade

The goal of this issue is to make sure that we have the automation work and process for upgrading documented so that it is easy for the next group to pick up future gitaly OS upgrades.

Location for upgrade notes: https://gitlab.com/gitlab-com/runbooks/-/tree/master/docs/gitaly

Specific items to cover:

1. This is where the scripts are stored
2. This is the testing we performed to know that the scripts are good
3. This is how to run the scripts
4. This is how you test it in staging
5. This is the epic link to where the previous upgrade was performed
6. These are the things we found helpful during the process (our recommendations)
7. The batch sizes we recommend

• Runbooks MR: gitlab-com/runbooks!4441 (merged)