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

…s#1687) (googleapis#380)

* 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://togithub.com/googleapis/synthtool/commit/d4b291604f148cde065838c498bc8aa79b8dc10e
Post-Processor: gcr.io/cloud-devrel-public-resources/owlbot-java:latest@sha256:edae91ccdd2dded2f572ec341a768ad180305a3e8fbfd93064b28e237d35920a