Deprecate testing utilities introduced in 3.10 in favor of graphql-testing-library

[jerelmiller]

We have moved the schema testing utilities introduced in 3.10 over to graphql-testing-library which is a client-agnostic tool. We will be recommending this going forward. As such, we plan to deprecate the testing utilities in 3.11 and remove them in 3.12.

[jerelmiller]

Finished with #11930

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