Use the shared recursion limit in diff()

diff_depth_limit() is gone in favor of detail::recursion_depth_limit().

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

tests/src/unit-json_patch.cpp

@@ -1813,7 +1813,7 @@ TEST_CASE("JSON patch: diff of deeply nested values")
     SECTION("the diff reproduces the target at every depth")
     {
         // every depth on either side of the nesting depth up to which diff()
-        // recurses (basic_json::diff_depth_limit(), 128)
+        // recurses (detail::recursion_depth_limit(), 128)
         for (std::size_t depth = 0; depth <= 300; ++depth)
         {
             CAPTURE(depth);