Functions to get a list of all CP releases and to get the migration URL from a release.

cp_versions_util.php

/**
 * Get a list of ClassicPress released versions from api-v1.classicpress.net.
 *
 * @return array|false Array of CP versions or false on API failure.
 */
function get_cp_versions() {

	$response = wp_remote_get('https://api-v1.classicpress.net/v1/upgrade/index.php', ['timeout'=>3]);
	if (is_wp_error($response) || empty($response)) {
		return false;
	}
	$versions = json_decode(wp_remote_retrieve_body($response));

	// Get only stable releases
	foreach ($versions as $key => $version) {
		if(!str_contains($version, 'nightly') && !str_contains($version, 'rc') && !str_contains($version, 'alpha') && !str_contains($version, 'beta')) {
			continue;
		}
		unset($versions[$key]);
	}

	// Strip .json from version
	$versions = array_map(
		function($v) {
			return substr($v, 0, -5);
		},
		$versions
	);

	// Sort using SemVer
	usort($versions, 'version_compare');

	return array_values($versions);

}

/**
 * Get a list of ClassicPress released versions from api.github.com.
 *
 * @return array|false URL for release. Empty string if no release found. False on API failure.
 */
function get_migration_from_cp_version($version) {
	$response = wp_remote_get('https://api.github.com/repos/ClassicPress/ClassicPress-release/releases/tags/'.$version, ['timeout'=>3]);
	if (is_wp_error($response) || empty($response)) {
		return false;
	}

	$data = json_decode(wp_remote_retrieve_body($response), true);
	if(isset($data['message']) && $data['message'] === 'Not Found') {
		return '';
	}

	$created_at = new \DateTime($data['created_at']);
	$day        = $created_at->format('Ymd');
	$exploded   = explode('.', $version);
	$major      = $exploded[0];
	$url        = 'https://github.com/ClassyBot/ClassicPress-v'.$major.'-nightly/releases/download/'.$version.'%2Bmigration.'.$day.'/ClassicPress-nightly-'.$version.'-migration.'.$day.'.zip';
	return $url;
}