you don’t need to go to 13.2.10 as in your post, you just need to go from 13.1.11 to the last 13.12.x release - I think 13.12.15 currently and then to 14.0.x.
You will need to reference the upgrade documentation and after each upgrade, make sure you check that background migrations have finished before starting the next upgrade. All background migrations must finish, and the command must return a 0.
No you have to jump through every version in the upgrade path. Otherwise you will most likely break your install when certain migrations are missed.
So your first upgrade is 11.11.8, then 12.0.12, and so on as per my previous post. Make sure you refer to the upgrade docs and make sure after each upgrade that you check that background migrations have finished before starting the next upgrade. Everything is in the docs. If you follow it, you cannot go wrong.
And make sure you have a backup of your server to restore if something does go wrong. Reading the docs and instructions is important to minimise the amount of problems later.
Yes of course, it can be run anytime. I ran mine now, works fine, reports zero. I don’t get any errors. If you are getting errors, it would really help to post the error message, since the above post doesn’t have anything to help diagnose the problem running that command.
If running that command as a normal user, try running sudo gitlab-rails instead.