kokoro: update tooling, fix warnings

Kokoro changes:
-  Linux: gcc-13, cmake-3.31.2
- Windows: python 3.12, cmake-3.31.2

* Disable warnings in gtest, gmock

This is required in order to build with GCC 13, and Apple Clang with warnings-as-errors

Bug: crbug.com/383538610

* disable license check on .github/workflows/main.yml

* Use add_subdirectory for internal gtest, gmock

---------

Co-authored-by: Antonio Maiorano <amaiorano@google.com>

kokoro/ubuntu/presubmit-docker.sh

@@ -28,8 +28,8 @@ git config --global --add safe.directory '*'
 git submodule update --init
 
 if [ "$BUILD_SYSTEM" == "cmake" ]; then
-    using cmake-3.17.2
-    using gcc-9
+    using cmake-3.31.2
+    using gcc-13
 
     mkdir build
     cd build