Drop support for python 3.6 and 3.7

[parthea]

Created a bug to track dropping support for python 3.6 and 3.7.

Once we drop python 3.6 and 3.7, we should update the ubuntu version to the latest in our github actions as suggested in googleapis/proto-plus-python#385

https://github.com/googleapis/proto-plus-python/blob/9d524420cde12a2994939a817c60647bf04e253a/.github/workflows/tests.yml#L42

[parthea]

Transferring to google-cloud-python as the code for proto-plus-python will be migrated there in 1-3 weeks.

[parthea]

Closing as duplicate of #16080