chore: update CI badge to point at Github Actions tests (#441)

Author: jackwotherspoon

README.md

@@ -7,7 +7,7 @@
 <h1 align="center">Cloud SQL Python Connector</h1>
 
 [![Open In Colab](https://colab.research.google.com/assets/colab-badge.svg)](https://colab.research.google.com/github/GoogleCloudPlatform/cloud-sql-python-connector/blob/main/samples/notebooks/postgres_python_connector.ipynb)
-![CI](https://storage.googleapis.com/cloud-devrel-public/cloud-sql-connectors/python/python3.10_linux.svg)
+[![CI](https://github.com/GoogleCloudPlatform/cloud-sql-python-connector/actions/workflows/tests.yml/badge.svg?event=push)](https://github.com/GoogleCloudPlatform/cloud-sql-python-connector/actions/workflows/tests.yml?query=event%3Apush+branch%3Amain)
 [![pypi](https://img.shields.io/pypi/v/cloud-sql-python-connector)](https://pypi.org/project/cloud-sql-python-connector)
 [![python](https://img.shields.io/pypi/pyversions/cloud-sql-python-connector)](https://pypi.org/project/cloud-sql-python-connector)