MDL-70457 forms: php74 fix for the (not used in core) date element

Curly brackets to access by index to strings or arrays is
deprecated since php74.

lib/pear/HTML/QuickForm/date.php

@@ -308,7 +308,7 @@ class HTML_QuickForm_date extends HTML_QuickForm_group
         $locale    =& $this->_locale[$this->_options['language']];
         $backslash =  false;
         for ($i = 0, $length = strlen($this->_options['format']); $i < $length; $i++) {
-            $sign = $this->_options['format']{$i};
+            $sign = $this->_options['format'][$i];
             if ($backslash) {
                 $backslash  = false;
                 $separator .= $sign;
@@ -507,4 +507,4 @@ class HTML_QuickForm_date extends HTML_QuickForm_group
 
     // }}}
 }
-?>
+?>