This PR migrates from using a private post processor image gcr.io/repo-automation-bots/owlbot-python:latest to a public one gcr.io/cloud-devrel-public-resources/owlbot-python:latest which is used by automation to update templated files across python repositories in the googleapis org.

Use the following commands to test the feature:

# Pull the latest python post processor image `gcr.io/cloud-devrel-public-resources/owlbot-python`
docker pull gcr.io/cloud-devrel-public-resources/owlbot-python:latest

# Read the digest of the post processor image `gcr.io/cloud-devrel-public-resources/owlbot-python` with the tag `latest`
docker inspect --format='{{.RepoDigests}}' gcr.io/cloud-devrel-public-resources/owlbot-python:latest 

# In the root of the repo, use this command to run the public post processor image 
docker run --user $(id -u):$(id -g) --rm -v $(pwd):/repo -w /repo gcr.io/cloud-devrel-public-resources/owlbot-python:latest

The benefit is that external maintainers should now be able to run the post-processor locally and PRs should continue to be opened by automation when there are updates to templated files.

I tested the functionality in the python-datastream repository and it seems to be working correctly. If there is any issue , we can roll back to the private post processor image.

This PR also removes obsolete replacements in owlbot.py from the master->main branch migration.