Remove use of Vulnerability.states[:somesymbol] from service classes

We can just use the enum states directly as symbols rather than
calling `.states` on the class

This was brought up in a comment on another MR ([here][0]):

> I noticed a lot of indexing and explicit references to the
> Vulnerability State enum which Rails will conveniently magic away for
> us anyways. I've added in suggestions to let Rails handle that
> behaviour more for us

This commit removes those aforementioned explicit usages of the enum

[0]:https://gitlab.com/gitlab-org/gitlab/-/merge_requests/99142#note_1118961906