Upgrade/Install: Rename wp_in_maintenance_mode() to wp_is_maintenance_mode(), for consistency with wp_is_recovery_mode().

While the former name might be a bit more accurate, the latter matches the existing naming pattern.

Follow-up to [47623].

Fixes #49959.
Built from https://develop.svn.wordpress.org/trunk@47871


git-svn-id: http://core.svn.wordpress.org/trunk@47646 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/class-wp-fatal-error-handler.php

@@ -31,7 +31,7 @@ class WP_Fatal_Error_Handler {
 		}
 
 		// Do not trigger the fatal error handler while updates are being installed.
-		if ( wp_in_maintenance_mode() ) {
+		if ( wp_is_maintenance_mode() ) {
 			return;
 		}