Test Image using unsupported Python version (3.7)

[msampathkumar]

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

Test image (.kokoro/docker/Dockerfile) 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!