update_nag()

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

	global $pagenow;

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

	$cur = get_preferred_from_update_core();

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

	if ( current_user_can( 'update_core' ) ) {
		$msg = sprintf(
			/* translators: 1: Codex URL to release notes, 2: new ClassicPress version, 3: URL to network admin, 4: accessibility text */
			__( '<a href="%1$s">ClassicPress %2$s</a> is available! <a href="%3$s" aria-label="%4$s">Please update now</a>.' ),
			sprintf(
				/* translators: %s: ClassicPress version */
				esc_url( __( 'https://www.classicpress.net/version/%s' ) ),
				$cur->current
			),
			$cur->current,
			network_admin_url( 'update-core.php' ),
			esc_attr__( 'Please update ClassicPress now' )
		);
	} else {
		$msg = sprintf(
			/* translators: 1: Codex URL to release notes, 2: new ClassicPress version */
			__( '<a href="%1$s">ClassicPress %2$s</a> is available! Please notify the site administrator.' ),
			sprintf(
				/* translators: %s: ClassicPress version */
				esc_url( __( 'https://www.classicpress.net/version/%s' ) ),
				$cur->current
			),
			$cur->current
		);
	}
	echo "<div class='update-nag'>$msg</div>";
}