chore(java): add a note in README for migrated split repos (#1687) (#2148)

* chore(java): add a note in README for migrated split repos

Disable renovate bot and flaky bot for split repositories
that have moved to the Java monorepo.
The Java monorepo will pass the "monorepo=True" parameter
to java.common_templates method in its owlbot.py files so that
the migration note will not appear in the README in the monorepo.

Co-authored-by: Jeff Ching <[email protected]>
Source-Link: https://github.com/googleapis/synthtool/commit/d4b291604f148cde065838c498bc8aa79b8dc10e
Post-Processor: gcr.io/cloud-devrel-public-resources/owlbot-java:latest@sha256:edae91ccdd2dded2f572ec341a768ad180305a3e8fbfd93064b28e237d35920a

Author: gcf-owl-bot[bot]

.github/.OwlBot.lock.yaml

@@ -13,4 +13,4 @@
 # limitations under the License.
 docker:
   image: gcr.io/cloud-devrel-public-resources/owlbot-java:latest
-  digest: sha256:bd316b9a591c45264775ecaba72e37d65905925c323b68e51a08c4126e4b6d0a
+  digest: sha256:edae91ccdd2dded2f572ec341a768ad180305a3e8fbfd93064b28e237d35920a