Validate/Lint that the trigger:project: must not be an array

We don't document that trigger:project can be an array but we in-consistently enforce that it can only be a string. This leads to a bug when the user specifies an array. See: https://gitlab.com/gitlab-com/ops-sub-department/section-ops-request-for-help/-/issues/25

Below are two equivalent syntaxes but both should be invalid:

1. invalid yaml according to the linter:

do:
  trigger: [allison.browne/multiproject2]

2. valid yaml according to the linter (but shouldn't be):

do:
  trigger: 
    project: [allison.browne/multiproject2]

Implementation

backend	MR
Add CI string validation to trigger:project	!107275 (merged)