Use 3.12 instead of 3.8 for system tests

Author: gkevinzheng

.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

@@ -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", "3.12"]
+SYSTEM_TEST_PYTHON_VERSIONS: List[str] = ["3.12"]
 SYSTEM_TEST_STANDARD_DEPENDENCIES: List[str] = [
     "mock",
     "pytest",
@@ -206,7 +206,6 @@ 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"):
         session.skip("cpp implementation is not supported in python 3.11+")