refactor(checkbox): remove legacy property and support for legacy syntax (#29043)

BREAKING CHANGE:

The `legacy` property and support for the legacy syntax, which involved placing an `ion-checkbox` inside of an `ion-item` with an `ion-label`, have been removed from checkbox. For more information on migrating from the legacy checkbox syntax, refer to the [Checkbox documentation](https://ionicframework.com/docs/api/checkbox#migrating-from-legacy-checkbox-syntax).

core/src/components/item/test/disabled/index.html

@@ -39,44 +39,12 @@
             <ion-label>Disabled Item Anchor</ion-label>
           </ion-item>
 
-          <ion-item>
-            <ion-checkbox disabled checked> Disabled Checkbox </ion-checkbox>
-          </ion-item>
-
           <ion-item>
             <ion-range disabled value="10">
               <span slot="label">Disabled Range</span>
             </ion-range>
           </ion-item>
         </ion-list>
-
-        <ion-list>
-          <ion-list-header>
-            <ion-label>Multiple Input Disabled Items</ion-label>
-          </ion-list-header>
-
-          <ion-item>
-            <ion-checkbox slot="start" aria-label="Checkbox"></ion-checkbox>
-            <ion-label>Checkbox + Range</ion-label>
-            <ion-range disabled value="45" aria-label="Range"></ion-range>
-          </ion-item>
-
-          <ion-item>
-            <ion-checkbox aria-label="Checkbox" slot="start"></ion-checkbox>
-          </ion-item>
-
-          <ion-item>
-            <ion-checkbox aria-label="Checkbox" disabled slot="start"></ion-checkbox>
-            <ion-label>Checkbox + Buttons</ion-label>
-            <ion-button slot="end">Default</ion-button>
-            <ion-button slot="end">Buttons</ion-button>
-          </ion-item>
-
-          <ion-item>
-            <ion-checkbox slot="start" disabled aria-label="Checkbox"></ion-checkbox>
-            <ion-searchbar></ion-searchbar>
-          </ion-item>
-        </ion-list>
       </ion-content>
 
       <ion-footer>

core/src/components/item/test/legacy/disabled/index.html

@@ -38,46 +38,12 @@
           <ion-item disabled href="#">
             <ion-label>Disabled Item Anchor</ion-label>
           </ion-item>
-          <ion-item>
-            <ion-label>Disabled Checkbox</ion-label>
-            <ion-checkbox disabled checked slot="start" legacy="true"></ion-checkbox>
-          </ion-item>
 
           <ion-item>
             <ion-label>Disabled Range</ion-label>
             <ion-range disabled value="10" legacy="true"></ion-range>
           </ion-item>
         </ion-list>
-
-        <ion-list>
-          <ion-list-header>
-            <ion-label>Multiple Input Disabled Items</ion-label>
-          </ion-list-header>
-
-          <ion-item>
-            <ion-checkbox disabled slot="start" legacy="true"></ion-checkbox>
-            <ion-label>Checkbox</ion-label>
-          </ion-item>
-
-          <ion-item>
-            <ion-checkbox slot="start" legacy="true"></ion-checkbox>
-            <ion-label>Checkbox + Range</ion-label>
-            <ion-range disabled value="45"></ion-range>
-          </ion-item>
-
-          <ion-item>
-            <ion-checkbox disabled slot="start" legacy="true"></ion-checkbox>
-            <ion-label>Checkbox + Buttons</ion-label>
-            <ion-button slot="end">Default</ion-button>
-            <ion-button slot="end">Buttons</ion-button>
-          </ion-item>
-
-          <ion-item>
-            <ion-checkbox slot="start" disabled legacy="true"></ion-checkbox>
-            <ion-label>Disabled Checkbox</ion-label>
-            <ion-searchbar></ion-searchbar>
-          </ion-item>
-        </ion-list>
       </ion-content>
 
       <ion-footer>