[4.0] Deprecate subscribeAndCount testing utility

[jerelmiller]

With the addition of the internal ObservableStream utility, we no longer have a need for subscribeAndCount. #12163 refactored existing tests to use ObservableStream instead.

Since this utility is listed in the changelog (see 3.7.0), we want to deprecate and remove it in 4.0.

[jerelmiller]

Done with #12223

[github-actions]

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