chore: require Python ≥ 3.9, protobuf ≥ 4.25.8 (#877)

Require Python 3.9+
Require protobuf 4.25.8+
Simplify code accordingly.

Fixes googleapis/google-cloud-python#15019 

This also required removing 3.7 and 3.8 as required checks in the repo
config, under branch protection rules.
29 files changed