chore: remove 3.9 tests from kokoro (googleapis#1293)

3.9 has been removed from the kokoro base image, so we should remove the
tests from our configs

3.9 is still tested in GitHub Actions

Author: daniel-sanche

.kokoro/presubmit/system.cfg

@@ -3,5 +3,5 @@
 # Only run this nox session.
 env_vars: {
     key: "NOX_SESSION"
-    value: "system-3.9"
-}
+    value: "system-3.10"
+}

noxfile.py

@@ -63,7 +63,7 @@
 UNIT_TEST_EXTRAS: List[str] = []
 UNIT_TEST_EXTRAS_BY_PYTHON: Dict[str, List[str]] = {}
 
-SYSTEM_TEST_PYTHON_VERSIONS: List[str] = ["3.9", "3.14"]
+SYSTEM_TEST_PYTHON_VERSIONS: List[str] = ["3.10", "3.14"]
 SYSTEM_TEST_STANDARD_DEPENDENCIES: List[str] = [
     "mock",
     "pytest",
@@ -83,7 +83,6 @@
 
 # 'docfx' is excluded since it only needs to run in 'docs-presubmit'
 nox.options.sessions = [
-    "unit-3.9",
     "unit-3.10",
     "unit-3.11",
     "unit-3.12",