feat: Add Python 3.14 support and drop Python 3.7/3.8

This commit adds support for Python 3.14 to the library and drops support for Python 3.7 and 3.8.

The following changes were made:
- Dropped support for Python 3.7 and 3.8 across all configuration files, documentation, and CI/CD pipelines.
- Added a 3.14 trove classifier in `setup.py`.
- Added a `grpcio` version constraint for Python 3.14 in `setup.py`.
- Added a 3.14 presubmit configuration file in `.kokoro/samples`.
- Added a `constraints-3.14.txt` file and moved it to the `testing/` directory.
- Updated the `CONTRIBUTING.rst` file to include 3.14 and remove 3.7/3.8.
- Updated the `README.rst` file to include 3.14 and remove 3.7/3.8.
- Updated `owlbot.py` to include "3.14" and remove "3.7"/"3.8" in `unit_test_python_versions`.
- Updated `noxfile.py` to include "3.14" and remove "3.7"/"3.8" in `UNIT_TEST_PYTHON_VERSIONS`.
- Updated `.github/sync-repo-settings.yaml` to include 3.14 and remove 3.7/3.8.
- Removed Kokoro and testing configurations for Python 3.7 and 3.8.
- Fixed a flaky test in `tests/unit/pubsub_v1/publisher/test_publisher_client.py`.
- Fixed an `AttributeError` in `google/cloud/pubsub_v1/subscriber/scheduler.py` that occurred in Python 3.14.

Author: google-labs-jules[bot]

.kokoro/samples/python3.14/common.cfg

@@ -16,7 +16,7 @@ env_vars: {
 # Declare build specific Cloud project.
 env_vars: {
     key: "BUILD_SPECIFIC_GCLOUD_PROJECT"
-    value: "python-docs-samples-tests-py314"
+    value: "python-docs-samples-tests-314"
 }
 
 env_vars: {

noxfile.py

@@ -233,12 +233,7 @@ def install_unittest_dependencies(session, *constraints):
 def unit(session, protobuf_implementation):
     # Install all test dependencies, then install this package in-place.
 
-    if protobuf_implementation == "cpp" and session.python in (
-        "3.11",
-        "3.12",
-        "3.13",
-        "3.14",
-    ):
+    if protobuf_implementation == "cpp" and session.python in ("3.11", "3.12", "3.13", "3.14"):
         session.skip("cpp implementation is not supported in python 3.11+")
 
     constraints_path = str(
@@ -448,12 +443,7 @@ def docfx(session):
 def prerelease_deps(session, protobuf_implementation):
     """Run all tests with prerelease versions of dependencies installed."""
 
-    if protobuf_implementation == "cpp" and session.python in (
-        "3.11",
-        "3.12",
-        "3.13",
-        "3.14",
-    ):
+    if protobuf_implementation == "cpp" and session.python in ("3.11", "3.12", "3.13", "3.14"):
         session.skip("cpp implementation is not supported in python 3.11+")
 
     # Install all dependencies