chore(eventstream-serde-node): use Readable.from for serializing

[trivikr]

Issue

N/A, noticed while browsing code.

Description

Removes the check for Readable.from as it's available in all Node.js supported by v3 (14+)
Docs: https://nodejs.org/api/stream.html#streamreadablefromiterable-options

Testing

The integration tests in transcribe streaming is successful

$ client-transcribe-streaming> node -v
v14.0.0

$ client-transcribe-streaming> yarn test:integration
yarn run v1.22.17
$ jest --config jest.integ.config.js
 PASS  test/index.integ.spec.ts (9.137 s)
  TranscribeStream client
    ✓ should stream the transcript (5081 ms)

Test Suites: 1 passed, 1 total
Tests:       1 passed, 1 total
Snapshots:   0 total
Time:        9.192 s
Ran all test suites.
Done in 10.03s

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