fix(button): update icon size to the default size (#29255)

Issue number: ROU-4815

---------
## What is the current behavior?
- Ionic theme, Icon size was 14px to the default button size;

## What is the new behavior?
- Ionic theme, Icon size has now 16px to the default button size;

## NOTE:
- Tests related... this a fix on top of [PR of ROU-4815
branch](https://github.com/ionic-team/ionic-framework/pull/29187) and as
was mentioned there, tests will be implemented under a task that will be
then created specifically for it.

core/src/components/button/button.ionic.scss

@@ -106,11 +106,22 @@
 // -------------------------------------------------------------------------------
 
 /* Button containing only an icon */
+::slotted(ion-icon[slot="start"]),
+::slotted(ion-icon[slot="end"]),
+::slotted(ion-icon[slot="icon-only"]) {
+  font-size: px-to-rem(16);
+}
+
+:host(.button-xsmall),
+:host(.button-small),
+:host(.button-large),
+:host(.button-xlarge) {
   ::slotted(ion-icon[slot="start"]),
   ::slotted(ion-icon[slot="end"]),
   ::slotted(ion-icon[slot="icon-only"]) {
     font-size: 1em;
   }
+}
 
 /* Button extra small */
 :host(.button-has-icon-only.button-xsmall) {