Add Operator impact assessment to Chart MRs acceptance criteria

GitLab Operator depends on GitLab Chart and due to this dependency some of the changes in Chart may impact Operator functionality and require follow-up work in Operator to incorporate the changes.

Acceptance criteria

• Define the relationship between GitLab Chart and Operator in more details in developer documents of GitLab Operator -> gitlab-org/cloud-native/gitlab-operator!659 (merged)
• Change Chart MR template and add Operator impact assessment as an acceptance criteria -> gitlab-org/charts/gitlab!3303 (merged)
• In Chart MR template, reference the Operator developer documents to clarify what may impact Operator -> gitlab-org/charts/gitlab!3303 (comment 1491011425)

Further work

• Add an issue template to GitLab Operator for Chart impact follow-up.