Add warning about removal of Update Change Dialog

Update Change Dialog was initially added to make it clear to a user
unafamiliar with the Gerrit workflow on how to modify the change.

This dialog isn't serving that purpose in a great way, it is not
perfectly clear to a new user that Update Change should give the
commands for it. There is already a section in the Documentation
dropdown for Uploading Change which should serve this purpose.

The overall usage of the button is fairly low(100-150 clicks per day).

Show a warning to the user that the button and the dialog will be
removed from Gerrit.

Screenshot: https://imgur.com/a/OzRoqZ2
Change-Id: Ib6405a68ded357a802c8888c413b8a432b2f4474
diff --git a/polygerrit-ui/app/elements/change/gr-upload-help-dialog/gr-upload-help-dialog_html.ts b/polygerrit-ui/app/elements/change/gr-upload-help-dialog/gr-upload-help-dialog_html.ts
index d633de5..9c27864 100644
--- a/polygerrit-ui/app/elements/change/gr-upload-help-dialog/gr-upload-help-dialog_html.ts
+++ b/polygerrit-ui/app/elements/change/gr-upload-help-dialog/gr-upload-help-dialog_html.ts
@@ -32,10 +32,24 @@
     p {
       margin-bottom: var(--spacing-m);
     }
+    .warning {
+      color: var(--warning-foreground);
+    }
   </style>
   <gr-dialog confirm-label="Done" cancel-label="" on-confirm="_handleCloseTap">
-    <div class="header" slot="header">How to update this change:</div>
+    <div class="header" slot="header">
+      How to update this change:
+    </div>
     <div class="main" slot="main">
+      <iron-icon
+        icon="gr-icons:warning"
+        class="warningBeforeSubmit"
+      ></iron-icon>
+      <span class="warning"
+        >The Update Change button will be removed from Gerrit in 2 weeks. You
+        can find the checkout command in the Download Dialog(keyboard shortcut:
+        d). We recommend making an alias for the other commands.
+      </span>
       <ol>
         <li>
           <p>