Move analytics to analytics.openneo.net

Self-hosted Plausible instance! I have need of usage numbers again,
after a good few years of just not using it; but I don't want to send
the data to Google, and I enjoy self-hosting things, so here we have it!

I actually forgot we were still sending the DTI 2020 numbers to
Plausible's cloud instance, oops! I stopped paying for it 😅 They do a
pretty good job anonymizing, so I'm not too worried about the practical
effects of that, but I'll be pleased to keep it more in-house going
forward.

I'm not sure why `async` isn't in the recommended tag anymore, but fine
by me!

pages/_document.tsx

@@ -47,10 +47,9 @@ class MyDocument extends Document {
             }}
           />
           <script
-            async
             defer
             data-domain="impress-2020.openneo.net"
-            src="https://plausible.io/js/plausible.js"
+            src="https://analytics.openneo.net/js/script.js"
             dangerouslySetInnerHTML={{ __html: `` }}
           />
           <link