Add caution to schema testing doc

[jerelmiller]

We will be removing the experimental testing utilities in v4 (#12595) now that we've launched GraphQL Testing Library. This PR adds a caution block to the beginning of the page that warns of the upcoming removal of the API.

Preview:

[changeset-bot]

⚠️ No Changeset found

Latest commit: 7b9b8c1

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[pkg-pr-new]

npm i https://pkg.pr.new/@apollo/client@12596

commit: 7b9b8c1

[github-actions]

size-limit report 📦

Path	Size
dist/apollo-client.min.cjs	41.34 KB (0%)
import { ApolloClient, InMemoryCache, HttpLink } from "@apollo/client" (CJS)	50.79 KB (0%)
import { ApolloClient, InMemoryCache, HttpLink } from "@apollo/client" (production) (CJS)	47.88 KB (0%)
import { ApolloClient, InMemoryCache, HttpLink } from "@apollo/client"	36.33 KB (0%)
import { ApolloClient, InMemoryCache, HttpLink } from "@apollo/client" (production)	33.73 KB (0%)
import { ApolloProvider } from "@apollo/client/react"	1.26 KB (0%)
import { ApolloProvider } from "@apollo/client/react" (production)	1.24 KB (0%)
import { useQuery } from "@apollo/client/react"	10.09 KB (0%)
import { useQuery } from "@apollo/client/react" (production)	9.02 KB (0%)
import { useLazyQuery } from "@apollo/client/react"	9.48 KB (0%)
import { useMutation } from "@apollo/client/react"	3.62 KB (0%)
import { useMutation } from "@apollo/client/react" (production)	2.84 KB (0%)
import { useSubscription } from "@apollo/client/react"	4.42 KB (0%)
import { useSubscription } from "@apollo/client/react" (production)	3.48 KB (0%)
import { useSuspenseQuery } from "@apollo/client/react"	5.89 KB (0%)
import { useSuspenseQuery } from "@apollo/client/react" (production)	4.55 KB (0%)
import { useBackgroundQuery } from "@apollo/client/react"	5.38 KB (0%)
import { useBackgroundQuery } from "@apollo/client/react" (production)	4.04 KB (0%)
import { useLoadableQuery } from "@apollo/client/react"	5.47 KB (0%)
import { useLoadableQuery } from "@apollo/client/react" (production)	4.12 KB (0%)
import { useReadQuery } from "@apollo/client/react"	3.43 KB (0%)
import { useReadQuery } from "@apollo/client/react" (production)	3.37 KB (0%)
import { useFragment } from "@apollo/client/react"	2.36 KB (0%)
import { useFragment } from "@apollo/client/react" (production)	2.31 KB (0%)

[svc-apollo-docs]

✅ Docs preview ready

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File Changes

0 new, 1 changed, 0 removed

* (developer-tools)/react/(latest)/development-testing/schema-driven-testing.mdx

Build ID: ec80f8f91a0adf74cf60da99

URL: https://www.apollographql.com/docs/deploy-preview/ec80f8f91a0adf74cf60da99

[Meschreiber]

Just a small suggestion to give users more precise information, but otherwise LGTM!