Enable Google Analytics

Prior to this change, the Hackney Google Analytics container ID was
passed to `docusaurus/plugin-google-analytics`, which _looks_ correct
but was deprecated in favour of `docusaurus/plugin-google-tag-manager`.
Analytics would have stopped working sometime in 2023.

This change updates our site to use the modern approach. The plugin
itself is bundled with the classic theme we're using, so only
a configuration change is required.

The containerId is shipped to the browser on every page render, so it's
not considered a secret.

Author: spikeheap

docusaurus.config.js

@@ -52,7 +52,7 @@ const config = {
         theme: {
           customCss: './src/css/custom.css',
         },
-        googleTagManager: {
+        gtag: {
           containerId: 'G-TB9MPCKBPC',
         },
       }),