[gtest] Drop custom-rolled heterogeneous comparator functors in favor of C++ standard ones

* Standard heterogeneous comparator functors such as `std::equal_to<>` and `std::less<>` [have been available since C++14](https://en.cppreference.com/w/cpp/utility/functional/less_void)
* Now that [C++14 is the minimum supported version of C++ in Googletest](https://github.com/google/oss-policies-info/blob/main/foundational-cxx-support-matrix.md), let's delete these duplications of the standard library

PiperOrigin-RevId: 515743068
Change-Id: I1563a2f94039c3a6688429298555545a922f6d7e

googlemock/include/gmock/gmock-matchers.h

@@ -257,6 +257,7 @@
 
 #include <algorithm>
 #include <cmath>
+#include <functional>
 #include <initializer_list>
 #include <ios>
 #include <iterator>
@@ -1199,27 +1200,27 @@ class PairMatchBase {
   };
 };
 
-class Eq2Matcher : public PairMatchBase<Eq2Matcher, AnyEq> {
+class Eq2Matcher : public PairMatchBase<Eq2Matcher, std::equal_to<>> {
  public:
   static const char* Desc() { return "an equal pair"; }
 };
-class Ne2Matcher : public PairMatchBase<Ne2Matcher, AnyNe> {
+class Ne2Matcher : public PairMatchBase<Ne2Matcher, std::not_equal_to<>> {
  public:
   static const char* Desc() { return "an unequal pair"; }
 };
-class Lt2Matcher : public PairMatchBase<Lt2Matcher, AnyLt> {
+class Lt2Matcher : public PairMatchBase<Lt2Matcher, std::less<>> {
  public:
   static const char* Desc() { return "a pair where the first < the second"; }
 };
-class Gt2Matcher : public PairMatchBase<Gt2Matcher, AnyGt> {
+class Gt2Matcher : public PairMatchBase<Gt2Matcher, std::greater<>> {
  public:
   static const char* Desc() { return "a pair where the first > the second"; }
 };
-class Le2Matcher : public PairMatchBase<Le2Matcher, AnyLe> {
+class Le2Matcher : public PairMatchBase<Le2Matcher, std::less_equal<>> {
  public:
   static const char* Desc() { return "a pair where the first <= the second"; }
 };
-class Ge2Matcher : public PairMatchBase<Ge2Matcher, AnyGe> {
+class Ge2Matcher : public PairMatchBase<Ge2Matcher, std::greater_equal<>> {
  public:
   static const char* Desc() { return "a pair where the first >= the second"; }
 };