Backend: Graphql mutation for CI Lint

Summary

We decided that it's better to move the CI Lint query to a mutation because it's more suitable than the query for the purposes of the feature.

Solution

• Create CI Lint Mutation
• Deprecate CI Lint Field
  • Instrument the CI field so we can remove it in 6 months minimum - will be done in https://gitlab.com/gitlab-org/gitlab/-/issues/547642+
  • A new issue needs to be created for removal to be scheduled in 18.7 - Remove QueryType.ciConfig field (#546186 - closed)