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.

Author: vchudnov-g

.github/workflows/unittest.yml

@@ -35,15 +35,10 @@ jobs:
         nox -s prerelease_deps
   unit:
     name: unit${{ matrix.option }}-${{ matrix.python }}
-    # TODO(https://github.com/googleapis/gapic-generator-python/issues/2303): use `ubuntu-latest` once this bug is fixed.
-    # Use ubuntu-22.04 until Python 3.7 is removed from the test matrix
-    # https://docs.github.com/en/actions/using-github-hosted-runners/using-github-hosted-runners/about-github-hosted-runners#standard-github-hosted-runners-for-public-repositories
-    runs-on: ubuntu-22.04
+    runs-on: ubuntu-latest
     strategy:
       matrix:
         python:
-          - "3.7"
-          - "3.8"
           - "3.9"
           - "3.10"
           - "3.11"