Admin page: remove alt attribute from decorative icon (#34715)

The Alt value isn't necessary here, since the icon doesn't bring any additional info on the page for folks using screen readers.

This has the added benefit of getting rid of a React warning:

```
Warning: Failed prop type: Invalid prop `alt` of type `object` supplied to `PlanIcon`, expected `string`.
PlanIcon@wp-content/plugins/jetpack/_inc/build/admin.js?ver=15a830895a6f6d1e3c39:44686:1
MyPlanCard@wp-content/plugins/jetpack/_inc/build/admin.js?ver=15a830895a6f6d1e3c39:51709:20
```

projects/plugins/jetpack/_inc/client/my-plan/my-plan-card/index.jsx

@@ -27,7 +27,7 @@ const MyPlanCard = ( {
 		<div className={ cardClassNames }>
 			<div className="my-plan-card__primary">
 				<div className="my-plan-card__icon">
-					{ productSlug && <PlanIcon plan={ productSlug } alt={ title } /> }
+					{ productSlug && <PlanIcon plan={ productSlug } alt="" /> }
 				</div>
 				<div className="my-plan-card__header">
 					{ title && <h2 className="my-plan-card__title">{ title }</h2> }

projects/plugins/jetpack/changelog/fix-admin-plan-icon-alt

@@ -0,0 +1,4 @@
+Significance: patch
+Type: other
+
+Admin Page: remove alt attribute from decorative icon.