chore: allow releases on previous major versions (#552)

* chore: allow releases on previous major versions

* 🦉 Updates from OwlBot post-processor

See https://github.com/googleapis/repo-automation-bots/blob/main/packages/owl-bot/README.md

Co-authored-by: Owl Bot <gcf-owl-bot[bot]@users.noreply.github.com>

.github/release-please.yml

@@ -1,2 +1,11 @@
 releaseType: python
 handleGHRelease: true
+# NOTE: this section is generated by synthtool.languages.python
+# See https://github.com/googleapis/synthtool/blob/master/synthtool/languages/python.py
+branches:
+- branch: v1
+  handleGHRelease: true
+  releaseType: python
+- branch: v0
+  handleGHRelease: true
+  releaseType: python

owlbot.py

@@ -219,6 +219,8 @@ def lint_setup_py(session):
 
 python.py_samples(skip_readmes=True)
 
+python.configure_previous_major_version_branches()
+
 s.replace(
     "samples/beam/noxfile.py",
     """INSTALL_LIBRARY_FROM_SOURCE \= os.environ.get\("INSTALL_LIBRARY_FROM_SOURCE", False\) in \(