Fix v2503 bugs (#763)

* Remove googletest from gitmodules.

Recommended way of using gtest is through FetchContent

* Add <sstream> to DOTGraph.cpp, which is missing on FreeBSD

Author: fabianbs96

.gitmodules

@@ -2,10 +2,6 @@
 	path = external/json
 	url = https://github.com/nlohmann/json.git
 	ignore = dirty
-[submodule "lib/googletest"]
-	path = external/googletest
-	url = https://github.com/google/googletest.git
-	branch = master
 [submodule "external/json-schema-validator"]
 	path = external/json-schema-validator
 	url = https://github.com/pboettch/json-schema-validator.git

lib/Utils/DOTGraph.cpp

@@ -18,11 +18,11 @@
 
 #include "nlohmann/json.hpp"
 
-#include <algorithm>
 #include <filesystem>
 #include <fstream>
 #include <iterator>
 #include <ostream>
+#include <sstream>
 #include <utility>
 
 namespace psr {