Change `vulnerability_state_transitions` factory from plural to `vulnerability_state_transition`

This was raised here !113619 (comment 1307143253)

The factory for vulnerability_state_transition should be singular instead of plural vulnerability_state_transitions

Implementation plan:

• Rename https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/spec/factories/vulnerabilities/state_transitions.rb#L4 name to vulnerability_state_transition from vulnerability_state_transitions
• Change all the reference of vulnerability_state_transitions in specs to vulnerability_state_transition to make sure all the specs are passing