Test Image using unsupported Python version (3.7)

## Is your feature request related to a problem? Please describe.

[Test image (.kokoro/docker/Dockerfile)](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/main/.kokoro/docker/Dockerfile#L120) is using 3.7, which is not supported as per https://devguide.python.org/versions/

This may encourage other users to continue to use 3.7. Also next year when 3.8 becomes unsupported version, there may be too many dependencies to handle at once. So I request a pro-active identification of dependent samples and upgrading it to 3.8 or higher versions.

## Describe the solution you'd like.

(1) Identify the samples which are marked as 3.7 dependent
(2) upgrade them to use later version(preferable higher version like 3.10 or 3.9)

Thanks!