GraphQL mutation to update `OrganizationUser`

Background

For #441246 (closed). We need a way to update the OrganizationUser model to change the role of users.

Proposal

Add a new mutation to update the OrganizationUser model.

Requirements

• Accepts access_level argument
• Returns errors based on https://docs.gitlab.com/ee/development/api_graphql_styleguide.html#failure-relevant-to-the-user if acces_level cannot be updated
• Only organization owners can change access level
• Organization owners should not be allowed to change their own access level if they are the last owner