Add support for new GitLab community advisories

[pombredanne]

See https://gitlab.com/gitlab-org/advisories-community/
This is MIT-licensed per https://gitlab.com/gitlab-org/advisories-community/-/blob/main/LICENSE

This is time-delayed (read: outdated) :

This is an MIT-licensed, time-delayed clone of the GitLab Advisory Database that contains all advisories from the GitLab Advisory Database that are older than 180 days.
For more information about the data format and for the contribution guidelines,
please refer to the README.md of the GitLab Advisory Database.

[pombredanne]

Reported by @tsteenbe

[julianthome]

@pombredanne we recently reduced the time delay with which https://gitlab.com/gitlab-org/security-products/gemnasium-db advisories are populated to https://gitlab.com/gitlab-org/advisories-community/ from 180 down to 30 days.

[pombredanne]

@julianthome re

we recently reduced the time delay

Thank you ++ !

[sschuberth]

Should this be added to https://github.com/nexB/vulnerablecode/blob/main/SOURCES.rst as well?