Automatically sync osv-schema version from submodule.

[michaelkedar]

We have this SCHEMA_VERSION variable that is used when we generate OSV records:

osv.dev/osv/models.py

Line 38 in 41bfd39

SCHEMA_VERSION = '1.7.3'

Having to remember to manually update this is not good. Updating the variable also means we need to update a few tests that are expecting the schema version in the output.

We should find a way to grab this from the osv-schema submodule we have checked out