Bootstrap 5: Override float attribute on legends

I don't fully understand why bootstrap added this in v5, as the
discussion on the issue tracker isn't clear. But we neither want
nor need this.

app/assets/stylesheets/common.scss

@@ -686,6 +686,11 @@ form {
   }
 }
 
+/* Stop bootstrap 5 from floating legends when they don't need to be */
+legend {
+  float: none;
+}
+
 /* Rules for the search and direction forms */
 
 header .search_forms,