Set referer policy of upgrade notifcation to "strict-origin-when-cross-origin"

- https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Referrer-Policy
- Since Django 3.1, all Wagtail admin responses have been given a referer-policy: same-origin header. See: https://chipcullen.com/django-3-referrer-policy-change/
- #8560

client/src/components/UpgradeNotification/index.js

@@ -17,7 +17,9 @@ const initUpgradeNotification = () => {
   const releasesUrl = 'https://releases.wagtail.io/latest.txt';
   const currentVersion = container.dataset.wagtailVersion;
 
-  fetch(releasesUrl).then(response => {
+  fetch(releasesUrl, {
+    referrerPolicy: 'strict-origin-when-cross-origin',
+  }).then(response => {
     if (response.status !== 200) {
       // eslint-disable-next-line no-console
       console.log(`Unexpected response from ${releasesUrl}. Status: ${response.status}`);