GraphQL: Migrate from the TimeoutMiddleware to the Timeout plugin

What does this MR do?

The GraphQL gem has unofficially deprecated the Graphql::Schema::TimeoutMiddleware class in favor of Interpreter support for timeouts.

Due to https://github.com/rmosolgo/graphql-ruby/pull/2622 the middleware has become more difficult to test (!26378 (comment 301960574)), so it may be a good time to migrate to the plugin style.

Related to #207278 (closed)

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Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

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• Label as security and @ mention @gitlab-com/gl-security/appsec
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