Replace CancelToken to AbortSignal (#19352)

app/javascript/mastodon/load_polyfills.js

@@ -26,6 +26,7 @@ function loadPolyfills() {
   // Edge does not have requestIdleCallback and object-fit CSS property.
   // This avoids shipping them all the polyfills.
   const needsExtraPolyfills = !(
+    window.AbortController &&
     window.IntersectionObserver &&
     window.IntersectionObserverEntry &&
     'isIntersecting' in IntersectionObserverEntry.prototype &&