Remove `GitlabSchema#execute`, and small tidy ups related to max limits

What does this MR do?

Graphql::Schema#execute calls #multiplex so we can remove our #execute.

How to setup and validate locally (strongly suggested)

1. Open a private browsing window so you are an anonymous user.
2. Change GitlabSchema::DEFAULT_MAX_COMPLEXITY to a low value, like 2
3. Execute a query on GraphiQL Explorer http://127.0.0.1:3000/-/graphql-explorer
4. The complexity limit should apply

---

1. Log in as an admin user
2. Change GitlabSchema::ADMIN_MAX_COMPLEXITY to a low value, like 3
3. Execute a query on GraphiQL Explorer http://127.0.0.1:3000/-/graphql-explorer
4. The complexity limit should apply

---

1. Log in as a non-admin user
2. Change GitlabSchema::AUTHENTICATED_MAX_COMPLEXITY to a low value, like 4
3. Execute a query on GraphiQL Explorer http://127.0.0.1:3000/-/graphql-explorer
4. The complexity limit should apply

Does this MR meet the acceptance criteria?

Conformity

• [-] I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• [-] I have added/updated documentation, or it's not needed. (Is documentation required?)
• [-] I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• [-] I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• [-] I have tested this MR in all supported browsers, or it's not needed.
• [-] I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.