Fix inconsistent font size for markdown preview on new PR view (#11565)

We use same method for new issue form and issue view, but was missing from new PR view making it one place where markdown preview is inconsistent in font size.

web_src/less/_repository.less

@@ -1318,6 +1318,10 @@
                 }
             }
         }
+
+        .markdown {
+            font-size: 14px;
+        }
     }
 
     .filter.dropdown .menu {