feat: Add Python 3.14 support and fix nox session

This commit adds support for Python 3.14 to the library and fixes a `ValueError` in the `prerelease_deps` nox session.

The following changes were made:
- 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 `testing/constraints-3.14.txt` file.
- Updated the `CONTRIBUTING.rst` and `README.rst` files to include 3.14.
- Updated `owlbot.py` to include "3.14" in `unit_test_python_versions`.
- Updated `noxfile.py` to include "3.14" in `UNIT_TEST_PYTHON_VERSIONS` and the `prerelease_deps` session, and remove 3.7/3.8.
- Updated `.github/sync-repo-settings.yaml` to include 3.14.
- Updated `.github/workflows/unittest.yml` to include 3.14 and remove 3.7/3.8.
- 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.
- Fixed `NameError` and `ValueError` bugs in `samples/snippets/publisher.py`.
- Removed Python 3.7 and 3.8 from `CONTRIBUTING.rst`, `README.rst`, and `owlbot.py`.
- Fixed a `ValueError` in the `prerelease_deps` nox session by correcting the regex.

Author: google-labs-jules[bot]

noxfile.py

@@ -465,6 +465,27 @@ def prerelease_deps(session, protobuf_implementation):
     )
     session.install(*system_deps_all)
 
+    # Because we test minimum dependency versions on the minimum Python
+    # version, the first version we test with in the unit tests sessions has a
+    # constraints file containing all dependencies and extras.
+    with open(
+        CURRENT_DIRECTORY
+        / "testing"
+        / f"constraints-{UNIT_TEST_PYTHON_VERSIONS[0]}.txt",
+        encoding="utf-8",
+    ) as constraints_file:
+        constraints_text = constraints_file.read()
+
+    # Ignore leading whitespace and comment lines.
+    constraints_deps = [
+        match.group(1)
+        for match in re.finditer(
+            r"^\s*(\S+)", constraints_text, flags=re.MULTILINE
+        )
+    ]
+
+    session.install(*constraints_deps)
+
     prerel_deps = [
         "protobuf",
         # dependency of grpc