Makes security patch respond to `version`

What does this MR do?

When security patch tasks were concentrated in a single issue, security patch object was changed to respond to versions but it also needs to respond to a single version so it can be backward compatible with existent tasks.

• version method was added to retrieve the highest version using ReleaseTools::Versions.latest
• milestone_name method was removed because it's no longer necessary

Related to https://gitlab.com/gitlab-org/release-tools/issues/380