Add bundler audit to CI

This commit adds a bundler-audit job on gitlab ci. With this step bundler-audit will:

• Check for vulnerable versions of gems in Gemfile.lock.
• Check for insecure gem sources (http://).

bundler-audit uses the ruby-advisory-db - a db for vulnerable ruby gems.

To see the current result of the check, see: https://0xacab.org/schleuder/schleuder-web/-/jobs/27702