Drop support for React 16

[jerelmiller]

Closes #12185

Drops support for React 16.

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@apollo/client	Major

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[github-actions]

size-limit report 📦

Path	Size
dist/apollo-client.min.cjs	40.64 KB (0%)
import { ApolloClient, InMemoryCache, HttpLink } from "dist/main.cjs"	50.02 KB (0%)
import { ApolloClient, InMemoryCache, HttpLink } from "dist/main.cjs" (production)	47.14 KB (0%)
import { ApolloClient, InMemoryCache, HttpLink } from "dist/index.js"	36.14 KB (0%)
import { ApolloClient, InMemoryCache, HttpLink } from "dist/index.js" (production)	33.54 KB (0%)
import { ApolloProvider } from "dist/react/index.js"	1.26 KB (0%)
import { ApolloProvider } from "dist/react/index.js" (production)	1.24 KB (0%)
import { useQuery } from "dist/react/index.js"	5.21 KB (0%)
import { useQuery } from "dist/react/index.js" (production)	4.29 KB (0%)
import { useLazyQuery } from "dist/react/index.js"	5.7 KB (0%)
import { useLazyQuery } from "dist/react/index.js" (production)	4.78 KB (0%)
import { useMutation } from "dist/react/index.js"	3.62 KB (0%)
import { useMutation } from "dist/react/index.js" (production)	2.84 KB (0%)
import { useSubscription } from "dist/react/index.js"	4.43 KB (0%)
import { useSubscription } from "dist/react/index.js" (production)	3.48 KB (0%)
import { useSuspenseQuery } from "dist/react/index.js"	5.51 KB (0%)
import { useSuspenseQuery } from "dist/react/index.js" (production)	4.17 KB (0%)
import { useBackgroundQuery } from "dist/react/index.js"	5.01 KB (0%)
import { useBackgroundQuery } from "dist/react/index.js" (production)	3.66 KB (0%)
import { useLoadableQuery } from "dist/react/index.js"	5.09 KB (0%)
import { useLoadableQuery } from "dist/react/index.js" (production)	3.74 KB (0%)
import { useReadQuery } from "dist/react/index.js"	3.41 KB (0%)
import { useReadQuery } from "dist/react/index.js" (production)	3.35 KB (0%)
import { useFragment } from "dist/react/index.js"	2.36 KB (0%)
import { useFragment } from "dist/react/index.js" (production)	2.31 KB (0%)