Fix Experimental UI for cookie based authentication (#7827)

We're using Prometheus behind a reverse proxy which authenticates everything based on cookies.
Without this attribute the cookie is not send with the request for the manifest.json

Signed-off-by: Stefan Bueringer <sbueringer@gmail.com>

web/ui/react-app/public/index.html

@@ -27,7 +27,7 @@
       manifest.json provides metadata used when your web app is added to the
       homescreen on Android. See https://developers.google.com/web/fundamentals/web-app-manifest/
     -->
-    <link rel="manifest" href="%PUBLIC_URL%/manifest.json" />
+    <link rel="manifest" href="%PUBLIC_URL%/manifest.json" crossorigin="use-credentials"/>
     <!--
       Notice the use of %PUBLIC_URL% in the tags above.
       It will be replaced with the URL of the `public` folder during the build.