### NOTE
* We shouldn't merge this PR until GCP cloud functions drops support for Python 3.7 ([Currently scheduled for GCF](https://cloud.google.com/functions/docs/runtime-support#python))
As part of supporting Python 3.12, we're now officially drop support for Python 3.7.
This PR:
* Changed supported Python version from 3.7 to 3.8 in README.
* Replaced distribution test image from `debian:buster` to `debian:bullseye` since the default Python version in buster is 3.7.
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Closes#34450
PiperOrigin-RevId: 622899511
The previous hack had bitrotted and was just returning 3.7. This method
was given to us by the GCF team and has backward compatibility
guarantees.
This will also help us to ensure that we don't accidentally remove
support for a particular Python runtime version before GCF does.
* Initial GCF distribtest
* Tenatively hook up to CI
* Try again
* Allow dev0 artifacts
* Fix invocation path
* Update gcloud
* Add a 3.8 artifact for presubmits
* And 3.9 too
* Put test files back to normal
* Formatting/linting
* Copyright
* That copyright script doesn't work with shebangs
* Review comments
* Try to create latest-manylinux label
* Accidentally a letter
* Add Python 3.7 manylinux 2014 to presubmit
* Revert CI config file used for test
* Review comments
* Yapf
* Re-add presubmit wheel
* Review comments
Xuan Wang