Replace std::random_shuffle with std::shuffle

According to [1], std::random_shuffle is deprecated in C++14 and removed
in C++17. Since std::shuffle is available since C++11 as a replacement
and monero already requires C++11, this is a good replacement.

A cryptographically secure random number generator is used in all cases
to prevent people from perhaps copying an insecure std::shuffle call
over to a place where a secure one would be warranted. A form of
defense-in-depth.

[1]: https://en.cppreference.com/w/cpp/algorithm/random_shuffle

tests/unit_tests/rolling_median.cpp

@@ -143,7 +143,7 @@ TEST(rolling_median, order)
     m.insert(random[i]);
   ASSERT_EQ(med, m.median());
 
-  std::shuffle(random.begin(), random.end(), std::default_random_engine(crypto::rand<unsigned>()));
+  std::shuffle(random.begin(), random.end(), crypto::random_device{});
   m.clear();
   for (int i = 0; i < 1000; ++i)
     m.insert(random[i]);