Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/43533

Switch to the new unified release workflow by default, now that this has been validated on the `0.74-stable` branch.

- Remove `--use-new-workflow` flag and remove legacy logic.
- Remove legacy `prepare_package_for_release` CI job, and use `run_new_release_workflow` -> `run_release_workflow` as new workflow condition match.

Changelog: [Internal]

Reviewed By: cortinico

Differential Revision: D55027120

fbshipit-source-id: 7c05cdff95ac369ce6cd1201ccfc5718798c4da6

# Conflicts:
#	scripts/releases-ci/prepare-package-for-release.js