Check DB versions before CERT severity updates

[timopollmeier]

What:

When rebuilding the SCAP or CERT database, check if the other one
has the correct version. If not, skip updating the CERT severity
scores.

Why:

This fixes an issue where the SCAP DB update during migration
does not work because the CERT DB is not migrated yet and therefore
incompatible.

How did you test it:

This was tested by running gvmd --migrate after the following:

• Set SCAP and CERT DB version to 1 ok
• Set SCAP DB version to 1, deleted CERT schema ok
• Deleted SCAP schema, set CERT schema to 1 ok
• Set SCAP DB version to 1, CERT DB version to 9999
• Set SCAP DB version to 9999, CERT DB version to 1

After each step, the database was fully synced/rebuilt by letting gvmd run
normally until the regular update finished.
After setting a high version number, it was also set to 1 after the test run
and migrated.

Checklist:

• Tests
• CHANGELOG Entry