Bump dartdoc to 0.34.0 (#66048)

diff --git a/dev/bots/docs.sh b/dev/bots/docs.sh
index 5fca152..010ba6a 100755
--- a/dev/bots/docs.sh
+++ b/dev/bots/docs.sh
@@ -37,7 +37,9 @@
 
 function generate_docs() {
     # Install and activate dartdoc.
-    "$PUB" global activate dartdoc 0.32.4
+    # NOTE: When updating to a new dartdoc version, please also update
+    # `dartdoc_options.yaml` to include newly introduced error and warning types.
+    "$PUB" global activate dartdoc 0.34.0
 
     # This script generates a unified doc set, and creates
     # a custom index.html, placing everything into dev/docs/doc.