Faced this issue when upgraded from focal/wallaby to focal/xena.

Nova API stopped working and was getting response as 500, found below errors in nova-conductor logs:
```
2023-08-14 03:39:07.655 103071 CRITICAL nova [req-c4bcb8f5-8e86-4efc-9097-8bd64f407c73 - - - - -] Unhandled error: nova.exception.TooOldComputeService: Current Nova version does not support computes older than Wallaby but the minimum compute service level in your system is 53 and the oldest supported service level is 54.
2023-08-14 03:39:07.655 103071 ERROR nova Traceback (most recent call last):
2023-08-14 03:39:07.655 103071 ERROR nova   File "/usr/bin/nova-conductor", line 10, in <module>
2023-08-14 03:39:07.655 103071 ERROR nova     sys.exit(main())
2023-08-14 03:39:07.655 103071 ERROR nova   File "/usr/lib/python3/dist-packages/nova/cmd/conductor.py", line 45, in main
2023-08-14 03:39:07.655 103071 ERROR nova     server = service.Service.create(binary='nova-conductor',
2023-08-14 03:39:07.655 103071 ERROR nova   File "/usr/lib/python3/dist-packages/nova/service.py", line 265, in create
2023-08-14 03:39:07.655 103071 ERROR nova     utils.raise_if_old_compute()
2023-08-14 03:39:07.655 103071 ERROR nova   File "/usr/lib/python3/dist-packages/nova/utils.py", line 1095, in raise_if_old_compute
2023-08-14 03:39:07.655 103071 ERROR nova     raise exception.TooOldComputeService(
2023-08-14 03:39:07.655 103071 ERROR nova nova.exception.TooOldComputeService: Current Nova version does not support computes older than Wallaby but the minimum compute service level in your system is 53 and the oldest supported service level is 54.
2023-08-14 03:39:07.655 103071 ERROR nova 
2023-08-14 03:39:11.252 103621 CRITICAL nova [req-d8227fbd-7a20-414f-8d4b-2a6f6354379a - - - - -] Unhandled error: nova.exception.TooOldComputeService: Current Nova version does not support computes older than Wallaby but the minimum compute service level in your system is 53 and the oldest supported service level is 54.
2023-08-14 03:39:11.252 103621 ERROR nova Traceback (most recent call last):
2023-08-14 03:39:11.252 103621 ERROR nova   File "/usr/bin/nova-conductor", line 10, in <module>
2023-08-14 03:39:11.252 103621 ERROR nova     sys.exit(main())
2023-08-14 03:39:11.252 103621 ERROR nova   File "/usr/lib/python3/dist-packages/nova/cmd/conductor.py", line 45, in main
2023-08-14 03:39:11.252 103621 ERROR nova     server = service.Service.create(binary='nova-conductor',
2023-08-14 03:39:11.252 103621 ERROR nova   File "/usr/lib/python3/dist-packages/nova/service.py", line 265, in create
2023-08-14 03:39:11.252 103621 ERROR nova     utils.raise_if_old_compute()
2023-08-14 03:39:11.252 103621 ERROR nova   File "/usr/lib/python3/dist-packages/nova/utils.py", line 1095, in raise_if_old_compute
2023-08-14 03:39:11.252 103621 ERROR nova     raise exception.TooOldComputeService(
2023-08-14 03:39:11.252 103621 ERROR nova nova.exception.TooOldComputeService: Current Nova version does not support computes older than Wallaby but the minimum compute service level in your system is 53 and the oldest supported service level is 54.
```

Below workaround resolved the problem: 

mysql -e "update nova.services set version = 56 where deleted = 0;"

Expectation:
This bug should be handled/resolved and nova api should continue to work after openstack release upgrades without any issues.

Thanks & regards