Fix serialize migration

Skip validation on the serialize migration. This might sound scary, but we only have the data we have when the migration happens. Presumably if a validation would fail it's because there is a new property that a future migration will fix. 

Fixes #8157 
Fixes #8141
Fixes #8184 
This will not solve the problem, then, will it? Migration from pre-7.3 to 7.4 will fail at this migration. The issue is the code says 'webhook' is required, but hasn't hit the migration to add it yet. That's why I think skipping validation is OK. The data that's there, is there. We can't do anything about it.