Get Versions Should Not Require Both Stable and Unstable Branches

The get_versions() function in the version_helper requires the version.json file to have both stable and unstable branch/objects.
https://github.com/phpbb/phpbb/blob/develop-ascraeus/phpBB/phpbb/version_helper.php#L250

This is fine for phpBB's usage. But if extensions want to use this functionality, the unstable branch should not be a requirement, so that support for more simple version.json files will be allowed, such as:

{
    "stable": {
        "**major.minor**": {
            "current": "**version information given**",
            "announcement": "https://www.phpbb.com/**link to announcement topic**",
            "eol": null,
            "security": **false|true**
        }
    }
}