Replace placeholder.com service with dummyimage (mdn#34568)

The domain driving placeholder.it and placeholder.com has been sold[1],
and the service is no longer available. Replace uses in MDN docs with
calls to dummyimage.com, which has a compatible API.

[1]: https://www.reddit.com/r/web_design/comments/10lqgja/placeholdercom_is_no_more/

files/en-us/web/css/css_grid_layout/grid_layout_and_progressive_enhancement/index.md

@@ -65,7 +65,7 @@ img {
 ```html
 <div class="media">
   <div class="image">
-    <img src="https://via.placeholder.com/150x150" alt="placeholder" />
+    <img src="https://dummyimage.com/150x150" alt="placeholder" />
   </div>
   <div class="text">
     This is a media object example. I am using floats for older browsers and

files/en-us/web/html/element/area/index.md

@@ -100,7 +100,7 @@ This element's attributes include the [global attributes](/en-US/docs/Web/HTML/G
 </map>
 <img
   usemap="#primary"
-  src="https://via.placeholder.com/350x150"
+  src="https://dummyimage.com/350x150"
   alt="350 x 150 pic" />
 ```