chore: cleanup cloud RAD generation (#1269)

Updating cloud rad generation to support [java-shared-config/pull/344](googleapis/java-shared-config#344) updates and removing inclusion of README as we're not currently supporting this feature yet
Source-Link: googleapis/synthtool@99ea92a
Post-Processor: gcr.io/repo-automation-bots/owlbot-java:latest@sha256:204b7af96e6d481f19b0ff377aa379d46bc56dd06e1cc7c523f361dd9cbfeeaa

Author: gcf-owl-bot[bot]

.github/.OwlBot.lock.yaml

@@ -1,3 +1,3 @@
 docker:
   image: gcr.io/repo-automation-bots/owlbot-java:latest
-  digest: sha256:fecf6bd85f19eb046d913982ea36f6d434f9a49ab0545d25e31186aa64367c0c
+  digest: sha256:204b7af96e6d481f19b0ff377aa379d46bc56dd06e1cc7c523f361dd9cbfeeaa

.kokoro/release/publish_javadoc11.sh

@@ -36,13 +36,9 @@ mvn clean install -B -q -DskipTests=true
 export NAME=google-cloud-bigquerymigration
 export VERSION=$(grep ${NAME}: versions.txt | cut -d: -f3)
 
-# V3 generates docfx yml from javadoc
-# generate yml
-mvn clean site -B -q -P docFX
-
-# copy README to docfx-yml dir and rename index.md
-cp README.md target/docfx-yml/index.md
-# copy CHANGELOG to docfx-yml dir and rename history.md
+# cloud RAD generation
+mvn clean javadoc:aggregate -B -q -P docFX
+# include CHANGELOG
 cp CHANGELOG.md target/docfx-yml/history.md
 
 pushd target/docfx-yml