Ensure the rails monolith can handle advisory metadata with up to 20 URLs

Relates to Ensure exported advisories comply with rails da... (#417409 - closed)

The advisory exporter can export advisory metadata with up to 20 URLs. We need to update the rails monolith which currently supports up to 10 URLs.

Background

According to this thread:

There's 662 advisories in gemnasium-db that have over 10 URLs, and there are 560 advisories with between 10 and 20 URLs.

Because of this, we've decided to increase the number of URLs supported by the advisory exporter to 20, and now we also need to update the rails db to handle this increased number, which is the purpose of this issue.

Implementation Plan

Only the pm_advisories table needs to be updated with the constraint changing from 10 to 20 via add_check_constraint.