Update dartdoc to 8.0.12 to fix focusing search field (flutter#151576)

Fixes dart-lang/dartdoc#3813 as 8.0.11
includes the fix in
dart-lang/dartdoc@346638e

Author: parlough

dev/bots/docs.sh

@@ -121,7 +121,7 @@ function generate_docs() {
     # Install and activate dartdoc.
     # When updating to a new dartdoc version, please also update
     # `dartdoc_options.yaml` to include newly introduced error and warning types.
-    "$DART" pub global activate dartdoc 8.0.10
+    "$DART" pub global activate dartdoc 8.0.12
 
     # Build and install the snippets tool, which resides in
     # the dev/docs/snippets directory.