Update `enableNewSmithyRuntime`'s default to orchestrator (#2924)

## Description
Updates the default value of `enableNewSmithyRuntime` in
`ClientCodegenConfig` to `orchestrator`.

----

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

Co-authored-by: ysaito1001 <awsaito@amazon.com>