Pin the `graphql` gem to `1.13.5`

What does this MR do and why?

The 1.13.6 release queries fields that aren't present in GitLab.com's current schema. See #876 (comment 822424264) and https://github.com/rmosolgo/graphql-ruby/pull/3854.

It's not clear to me yet if a < 1.13.6 constraint should be added to the gitlab-triage gem directly...

Action items

• (If applicable) Add documentation to the handbook pages for Triage Operations =>
• (If applicable) Identify the affected groups and how to communicate to them:
  • /cc @person_or_group =>
  • Relevant Slack channels =>
  • Engineering week-in-review

Closes #876 (closed).