Fix event handling on error

The delete-project UI component did not use the legacy mixin anymore,
but still used the this.fire() method provided by the mixin.

Bug: Issue 13402
Change-Id: Iac7779d2206e9ac8aa14ba4400c4c46eb04aba50

gr-delete-repo/gr-delete-repo.js

diff --git a/gr-delete-repo/gr-delete-repo.js b/gr-delete-repo/gr-delete-repo.js
index 9afef3c..20c3555 100644
--- a/gr-delete-repo/gr-delete-repo.js
+++ b/gr-delete-repo/gr-delete-repo.js
@@ -64,7 +64,11 @@
     };
 
     const errFn = response => {
-      this.fire('page-error', {response});
+      this.dispatchEvent(new CustomEvent('page-error', {
+        detail: {response},
+        bubbles: true,
+        composed: true,
+      }));
     };
 
     return this.plugin.restApi().send(