Update Pocket GTM Container (fixes #12624)

We have moved Pocket's GA4 property under the Mozilla Account.
We use a lookup table in the GTM container to set measurement ID
based on whether the hostname is production or not.
If production, the measurement ID connects to getpocket.com - GA4
property.
If not production, the measurement ID connects to getpocket.com -
GA4 Testing property.

gcp/bedrock-demos/cloudrun/pocket-demo.env.yaml

@@ -13,7 +13,7 @@ DB_DOWNLOAD_IGNORE_GIT: "True"
 DEBUG: "False"
 DEV: "True"
 GOOGLE_ANALYTICS_ID: "UA-370613-9"
-GTM_CONTAINER_ID: "G-NFR9Y40GD3"
+GTM_CONTAINER_ID: "GTM-P4LPJ42"
 LOG_LEVEL: INFO
 PROD_DETAILS_STORAGE: product_details.storage.PDDatabaseStorage
 RUN_SUPERVISOR: "True"

media/js/pocket/analytics.es6.js

@@ -11,7 +11,7 @@ const PocketAnalytics = {
     loaded: false,
 
     loadGA: () => {
-        // Google Tag Manager used to load GA4 (gtag.js)
+        // Google Tag Manager used to load GA4 (gtm.js)
         const GTM_CONTAINER_ID = document
             .getElementsByTagName('html')[0]
             .getAttribute('data-gtm-container-id');
@@ -21,15 +21,19 @@ const PocketAnalytics = {
         };
 
         if (GTM_CONTAINER_ID) {
-            const gaScript = document.createElement('script');
-            gaScript.async = 'true';
-            gaScript.type = 'text/javascript';
-            gaScript.src = `https://www.googletagmanager.com/gtag/js?id=${GTM_CONTAINER_ID}`;
-            const pageScript = document.getElementsByTagName('script')[0];
-            pageScript.parentNode.insertBefore(gaScript, pageScript);
-
-            window.gtag('js', new Date());
-            window.gtag('config', GTM_CONTAINER_ID);
+            (function (w, d, s, l, i) {
+                w[l] = w[l] || [];
+                w[l].push({
+                    'gtm.start': new Date().getTime(),
+                    event: 'gtm.js'
+                });
+                const f = d.getElementsByTagName(s)[0],
+                    j = d.createElement(s),
+                    dl = l !== 'dataLayer' ? '&l=' + l : '';
+                j.async = true;
+                j.src = 'https://www.googletagmanager.com/gtm.js?id=' + i + dl;
+                f.parentNode.insertBefore(j, f);
+            })(window, document, 'script', 'dataLayer', [GTM_CONTAINER_ID]);
         }
 
         // Google Universal Analytics (analytics.js)