Provide compatibility for backups pre-6.9

[jclarkeSTFC]

Provide compatibility for backups pre-6.9

Summary of work

If the new backup file is not detected, then use the old code for loading backups.

Allows 6.8 backups (and earlier versions) to work in 6.9.
Fixes #36900

To test:

See #36859 for further details.

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[jhaigh0]

When loading a 6.9 recovery in 6.8, I experienced the bug where Mantid runs the old cached version of ordered_recovery.py. I checked to make sure the right script had been generated by opening the file else where and it had. Not much we can do about that bug.

Everything else worked as expected

[thomashampson]

This fixes the new bug where mantid 6.9.0 wouldn't open after a 6.8.0 crash, and also improves the functionality so that project recovery is compatible in both directions (6.9 -> pre-6.9 and vice versa).