build: restore DEFAULT_PYTHON_VERSION and SYSTEM_TEST_PYTHON_VERSIONS to 3.14

- Fixes 'Python interpreter 3.10 not found' error in GitHub Actions lint job.

- Fixes 'system' nox session failing because GitHub Actions only provisions Python 3.14 for it.

Author: aseering

noxfile.py

@@ -36,16 +36,10 @@
 ISORT_VERSION = "isort==5.11.0"
 LINT_PATHS = ["google", "tests", "noxfile.py", "setup.py"]
 
-DEFAULT_PYTHON_VERSION = "3.10"
+DEFAULT_PYTHON_VERSION = "3.14"
 
 DEFAULT_MOCK_SERVER_TESTS_PYTHON_VERSION = "3.12"
-SYSTEM_TEST_PYTHON_VERSIONS: List[str] = [
-    "3.10",
-    "3.11",
-    "3.12",
-    "3.13",
-    "3.14",
-]
+SYSTEM_TEST_PYTHON_VERSIONS: List[str] = ["3.14"]
 
 UNIT_TEST_PYTHON_VERSIONS: List[str] = [
     "3.10",