Returns core update notification message.

function update_nag() {
	global $pagenow;

	if ( is_multisite() && ! current_user_can( 'update_core' ) ) {
		return false;
	}

	if ( 'update-core.php' === $pagenow ) {
		return;
	}

	$cur = get_preferred_from_update_core();

	if ( ! isset( $cur->response ) || 'upgrade' !== $cur->response ) {
		return false;
	}

	$version_url = sprintf(
		/* translators: %s: WordPress version. */
		esc_url( __( 'https://wordpress.org/support/wordpress-version/version-%s/' ) ),
		sanitize_title( $cur->current )
	);

	if ( current_user_can( 'update_core' ) ) {
		$msg = sprintf(
			/* translators: 1: URL to WordPress release notes, 2: New WordPress version, 3: URL to network admin, 4: Accessibility text. */
			__( 'WordPress %2$s is available! Please update now.' ),
			$version_url,
			$cur->current,
			network_admin_url( 'update-core.php' ),
			esc_attr__( 'Please update WordPress now' )
		);
	} else {
		$msg = sprintf(
			/* translators: 1: URL to WordPress release notes, 2: New WordPress version. */
			__( 'WordPress %2$s is available! Please notify the site administrator.' ),
			$version_url,
			$cur->current
		);
	}

	echo "
$msg
";
}