fix(ui): apply className only to root element in ButtonIcon

Remove duplicate className application from inner elements (content and spinner).
The className prop should only be applied to the root button element, matching
the behavior of the Button component.

Signed-off-by: Johan Persson <johanopersson@gmail.com>

.changeset/common-coins-stare.md

@@ -0,0 +1,7 @@
+---
+'@backstage/ui': patch
+---
+
+Fixed `ButtonIcon` incorrectly applying `className` to inner elements instead of only the root element.
+
+Affected components: ButtonIcon

.patches/pr-31900.txt

@@ -0,0 +1 @@
+Fix incorrectly applying className to three elements internally in ButtonIcon.

packages/ui/src/components/ButtonIcon/ButtonIcon.tsx

@@ -64,7 +64,6 @@ export const ButtonIcon = forwardRef(
                 classNamesButtonIcon.content,
                 stylesButton[classNames.content],
                 stylesButtonIcon[classNamesButtonIcon.content],
-                className,
               )}
             >
               {icon}
@@ -79,7 +78,6 @@ export const ButtonIcon = forwardRef(
                   classNamesButtonIcon.spinner,
                   stylesButton[classNames.spinner],
                   stylesButtonIcon[classNamesButtonIcon.spinner],
-                  className,
                 )}
               >
                 <RiLoader4Line aria-hidden="true" />