Remove V0 nightly download path from ios-prebuild (#56759)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/56759

The legacy Hermes V0 nightly dist-tag ('nightly') has been removed. This commit
cleans up the ios-prebuild Hermes download logic:
- Remove the 'nightly' sentinel string and getNightlyVersionFromNPM()
- Remove DOWNLOAD_PREBUILT_NIGHTLY_TARBALL source type and downloadPrebuiltNightlyTarball()
- Remove getNightlyTarballUrl() (was fetching from Maven Snapshots for V0 nightlies)
- Rename getLatestV1VersionFromNPM -> getLatestHermesVersionFromNPM
- Update prepare-app-utils.js to use 'latest-v1' instead of 'nightly' for HERMES_VERSION

## Changelog:
[Internal] -

Reviewed By: cortinico

Differential Revision: D104649632

fbshipit-source-id: 91f7c67ee59a3ef835d78146e5ad5ecab25a17bf