Upgrade dartdoc to 0.30.1 (#50980)

dev/bots/docs.sh

diff --git a/dev/bots/docs.sh b/dev/bots/docs.sh
index d9e2924..b11ffb2 100755
--- a/dev/bots/docs.sh
+++ b/dev/bots/docs.sh
@@ -111,7 +111,7 @@
 fi
 
 # Install and activate dartdoc.
-"$PUB" global activate dartdoc 0.29.3
+"$PUB" global activate dartdoc 0.30.1
 
 # This script generates a unified doc set, and creates
 # a custom index.html, placing everything into dev/docs/doc.