Use GA4 init code

- runs the new GA4 init code, which starts the analytics code only after cookie consent
- note that this will not run anywhere but integration because the code it refers to is only included on integration
- remove test that checked for the existing of the Google Tag Manager component, as we're not using it anymore

app/assets/javascripts/analytics.js.erb

@@ -144,3 +144,15 @@ window.GOVUK.analyticsVars.linkedDomains = linkedDomains
 if (typeof window.GOVUK.analyticsInit !== 'undefined') {
   window.GOVUK.analyticsInit()
 }
+
+<% if ENV["GOOGLE_TAG_MANAGER_ID"] %>
+  if (typeof window.GOVUK.analyticsGA4.init !== 'undefined') {
+    window.GOVUK.analyticsGA4 = window.GOVUK.analyticsGA4 || {}
+    window.GOVUK.analyticsGA4.vars = window.GOVUK.analyticsGA4.vars || {}
+    window.GOVUK.analyticsGA4.vars.id = '<%= ENV["GOOGLE_TAG_MANAGER_ID"] %>'
+    window.GOVUK.analyticsGA4.vars.auth = '<%= ENV["GOOGLE_TAG_MANAGER_AUTH"] %>'
+    window.GOVUK.analyticsGA4.vars.preview = '<%= ENV["GOOGLE_TAG_MANAGER_PREVIEW"] %>'
+
+    window.GOVUK.analyticsGA4.init()
+  }
+<% end %>

app/views/root/_error_page.html.erb

@@ -11,16 +11,6 @@
   global_bar = render "components/global_bar"
 %>
 
-<% content_for :head do %>
-  <% if ENV["GOOGLE_TAG_MANAGER_ID"] %>
-     <%= render "govuk_publishing_components/components/google_tag_manager_script", {
-       gtm_id: ENV["GOOGLE_TAG_MANAGER_ID"],
-       gtm_auth: ENV["GOOGLE_TAG_MANAGER_AUTH"],
-       gtm_preview: ENV["GOOGLE_TAG_MANAGER_PREVIEW"]
-     } %>
-  <% end %>
-<% end %>
-
 <%= render "govuk_publishing_components/components/layout_for_public", {
   emergency_banner: emergency_banner,
   full_width: false,

app/views/root/_gem_base.html.erb

@@ -30,16 +30,6 @@
   global_bar << global_banner if global_banner
 %>
 
-<% content_for :head do %>
-  <% if ENV["GOOGLE_TAG_MANAGER_ID"] %>
-     <%= render "govuk_publishing_components/components/google_tag_manager_script", {
-       gtm_id: ENV["GOOGLE_TAG_MANAGER_ID"],
-       gtm_auth: ENV["GOOGLE_TAG_MANAGER_AUTH"],
-       gtm_preview: ENV["GOOGLE_TAG_MANAGER_PREVIEW"]
-     } %>
-  <% end %>
-<% end %>
-
 <%= render "govuk_publishing_components/components/layout_for_public", {
   account_nav_location: account_nav_location,
   draft_watermark: draft_environment,