[Breaking change] 7.9.2-> 7.10 / 8.0 Upgrade fails (as tested via cloud staging)

[EricDavisX]

Change description

TBD. I expect its a break in 7.x / 7.10 / 8.0 that is causing this.

Reproduction steps
Install 7.9.2 latest BC on cloud staging
Use the Cloud 'upgrade' button to upgrade and select 7.10
Then wait 18 mins or so, and see it fail.

• details. I was using AWS US East (N. Virginia) and I had picked 'Elastic Security' as the template for deploy settings

Describe the change. How will it manifest to users?
I believe all 7.x cloud users will experience this when upgrading, but I don't know that for sure.

I take this as an internal dev issue that is sure to be fixed before 7.10 ships so more data should be immaterial, except as needed to debug and isolate the problem. And I need help to do that so... I'll leave this logged with minimal details I am confident in.

I am seeing ML and Ingest Manager permissions errors when I try to log in to Kibana, but I don't know if its a false positive as I don't know what state the system is in after the failed (partial?) upgrade

tbd if these are real, or really related:

[EricDavisX]

the 7.10 cloud staging deploy is failing as well, it may yet be that the upgrade cannot be tested until that is fixed and this issue is prematurely logged. i'll try a few more deploys to check it

[EricDavisX]

I believe I got caught by this Kibana oom issue when testing this, #76783

7.9.x seems not to have this... so you can do an install with defaults, but then when you upgrade 7.10 requires more memory and so will fail - making us think it was the upgrade, but its not, its just the current 7.10 builds.
:(

at least I can close this and we're one step closer to testing upgrade, when the feature work is all in.