Remove V0 nightly download path from hermes-utils.rb (#56755)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/56755

Remove the legacy Hermes V0 nightly (Maven Snapshots) download path:
- Remove DOWNLOAD_PREBUILT_NIGHTLY_TARBALL source type constant
- Remove nightly_artifact_exists(), nightly_tarball_url(), podspec_source_download_prebuilt_nightly_tarball()
- Remove net/http and rexml/document requires (only needed for nightly XML parsing)
- Rename BUILD_FROM_GITHUB_MAIN -> BUILD_FROM_GITHUB_STABLE_BRANCH (was always targeting the stable V1 branch)
- Rename force_build_from_main -> force_build_from_stable_branch
- Rename podspec_source_build_from_github_main -> podspec_source_build_from_github_stable_branch
- Extract HERMES_STABLE_BRANCH constant so the hardcoded branch is in one place

## Changelog:
[Internal] -

Reviewed By: cortinico

Differential Revision: D104649630

fbshipit-source-id: f0d0834b72ee77183276dba0058c49a0308d1158