Relocate and refactor set-rn-version script (#42730)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/42730

Reviewing and modernising this script as part of simplifying our release publish workflow.

- Drop unused `--dependency-versions` arg from CLI entry point
- Simplify templating approach
- Type as Flow
- Drop dependencies on `shelljs` and `yargs`
- Relocate under `scripts/releases/`
- Rewrite tests as snapshot tests

Changelog: [Internal]

Reviewed By: NickGerleman

Differential Revision: D53001971

fbshipit-source-id: e55a71a0bb37e3e18ba1e582a5c46ddd58823d81