ValueIterator operators -> and * must be const

[BillyDonahue]

Describe the bug

As discovered in #1216, The C++20 std::reverse_iterator<ValueIterator> cannot perform ->, at least not in GCC.
This is because ValueIterator operators -> and * are const-incorrect.

To Reproduce

Json::Value v;
v["k1"] = "v1";
std::reverse_iterator it(value.end());
auto s = it->asString();  // This fails to compile, but `(*it).asString()`; is fine.

Expected behavior

operator-> and operator* should be const in ValueIterator as they are in ValueConstIterator.
Dereferencing an iterator is always a logical const operation, as providing such access does not change the iterator.

Desktop (please complete the following information):

• OS: in my case, MacOS, using homebrew GCC-10.
• Meson version N/A
• Ninja version N/A

Additional context

A commit was made for this on #1216, but we could make it a standalone bugfix independent of C++20.
14e15b6