This relates to RHEL-84351

We probably need to backport a patch probably back to 9.4.z that handles the fallout from https://github.com/ostreedev/ostree/pull/3346

Here's what I'm thinking:

• Add a CI test to ostree upstream that makes a repo with the ostree-under-test, but uses e.g. the ostree from multiple older OSes (debian stable say) and verifies that `ostree fsck` passes. It should - except with a mix of unsorted xattrs (TODO verify)
• If that reproduces (time to file this as an upstream tracker) then we are going to have to do something really painful like when we get a checksum mismatch, try loading the object and swap the xattr order around and redo the checksum and accept if that passes
• Then, we'll need to ship that updated ostree widely

That will get us "new ostree works with old repos". However...for the bootc case of course it still doesn't work because it's the old ostree we're upgrading from.

So for RHEL we may need this to go back to 9.4.z at least