Bump ext/googletest to v1.13.0
The latest release of googletest requires C++14.
diff --git a/ext/googletest.cmd b/ext/googletest.cmd
index 5528dec..073b8ee 100755
--- a/ext/googletest.cmd
+++ b/ext/googletest.cmd
@@ -7,7 +7,7 @@
: # If you're running this on Windows, be sure you've already run this (from your VC2019 install dir):
: # "C:\Program Files (x86)\Microsoft Visual Studio\2019\Professional\VC\Auxiliary\Build\vcvars64.bat"
-git clone -b release-1.12.1 --depth 1 https://github.com/google/googletest.git
+git clone -b v1.13.0 --depth 1 https://github.com/google/googletest.git
cd googletest
mkdir build
cd build