chore: drop support for Ubuntu 16.04

As of April 2021, it will no longer be an LTS release, and thus us supporting it will increase maintenance burdens.

Also switch CI to use 18.04, so as to test what we currently support.

Closes #4505

docs/home.html

@@ -140,7 +140,7 @@
           <p class="line">
             <span class="path"></span>
             <span class="prompt">$</span>
-            <span class="command">echo "xenial bionic focal" | grep -q "$OS_ID" || OS_ID="bionic"</span>
+            <span class="command">echo "bionic focal" | grep -q "$OS_ID" || OS_ID="bionic"</span>
           </p>
           <p class="line">
             <span class="path"></span>