fix(clients): re-enable default http2 session concurrency

[kuhe]

Issue

#3809 😐

Description

Enable http2 session concurrency by default.

This does not apply to event stream operations - those use isolated http2 sessions in Node.js.

Testing

CI and recently added e2e tests in

• test(clients): add snapshot tests for http2-default clients #7947
• test(client-transcribe-streaming): add concurrency to startStreamTranscription test #7948
• test(client-kinesis): http2 session concurrency tests #7941

Checklist

• If the PR is a feature, add integration tests (*.integ.spec.ts) or E2E tests.
  • It's not a feature.
• My E2E tests are resilient to concurrent i/o.
  • I didn't write any E2E tests.
• I added access level annotations e.g. @public, @internal tags and enabled doc generation on the package. Remember that access level annotations go below the description, not above.
  • I didn't add any public functions.
• Streams - how do they work?? My WebStream readers/locks are properly lifecycled. Node.js stream backpressure is handled. Error handling.
  • No streams here.

---

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[kuhe]

we should start with a low default concurrency like ... 4