feat(ci): Update system tests to Python 3.12 and prep for Kokoro session cleanup

Author: chalmerlowe

.kokoro/presubmit/system-3.8.cfg .kokoro/presubmit/system-3.12.cfg.kokoro/presubmit/system-3.8.cfg renamed to .kokoro/presubmit/system-3.12.cfg

@@ -3,5 +3,5 @@
 # Only run this nox session.
 env_vars: {
     key: "NOX_SESSION"
-    value: "system-3.8"
-}
+    value: "system-3.12"
+}

noxfile.py

@@ -32,7 +32,7 @@
 ISORT_VERSION = "isort==5.11.0"
 LINT_PATHS = ["docs", "google", "tests", "noxfile.py", "setup.py"]
 
-DEFAULT_PYTHON_VERSION = "3.8"
+DEFAULT_PYTHON_VERSION = "3.12"
 
 UNIT_TEST_PYTHON_VERSIONS: List[str] = [
     "3.8",
@@ -58,7 +58,7 @@
 UNIT_TEST_EXTRAS: List[str] = []
 UNIT_TEST_EXTRAS_BY_PYTHON: Dict[str, List[str]] = {}
 
-SYSTEM_TEST_PYTHON_VERSIONS: List[str] = ["3.8"]
+SYSTEM_TEST_PYTHON_VERSIONS: List[str] = ["3.12"]
 SYSTEM_TEST_STANDARD_DEPENDENCIES: List[str] = [
     "mock",
     "pytest",