Replace at-/de-tached with dis-connectedCallback

attached() and detached() are Polymer 1 legacy.

Change-Id: Ic86d07bfb95ec316880db261a913004f5a6e5a22
diff --git a/polygerrit-ui/app/elements/diff/gr-diff-mode-selector/gr-diff-mode-selector.ts b/polygerrit-ui/app/elements/diff/gr-diff-mode-selector/gr-diff-mode-selector.ts
index 0a22556..cc7bb98 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff-mode-selector/gr-diff-mode-selector.ts
+++ b/polygerrit-ui/app/elements/diff/gr-diff-mode-selector/gr-diff-mode-selector.ts
@@ -45,7 +45,9 @@
 
   private readonly restApiService = appContext.restApiService;
 
-  attached() {
+  /** @override */
+  connectedCallback() {
+    super.connectedCallback();
     ((IronA11yAnnouncer as unknown) as FixIronA11yAnnouncer).requestAvailability();
   }