Volatile is deprecated in C++20

[Troels51]

Describe the bug
In C++20 a lot of the volatile keyword is being deprecated (P1152R3)
For example deallocate has this problem

To Reproduce
Steps to reproduce the behavior:

1. Use a new compiler, fx gcc10
2. Set CMAKE_CXX_STANDARD to 20
3. Build

Expected behavior
Expected the build to complete

Desktop (please complete the following information):

• gcc version 10.1.0

Additional context
I ran into this problem because i am using jsoncpp in a project that was setting CXX_STANDARD to 20, and then when we upgraded gcc we were getting compile errors.
For example

jsoncpp-src/include/json/allocator.h:41:36: error: ‘volatile’-qualified parameter is deprecated [-Werror=volatile]
   41 |   void deallocate(volatile pointer p, size_type n) {
      |                   ~~~~~~~~~~~~~~~~~^

[dota17]

I think we are far from ready to adapt to the C + + 20 standard. But if you like, you can submit a PR to fix this, and I'd love to review it then.

[cdunn2001]

@dawesc, any opinion? This was originally your code, apparently.

https://en.cppreference.com/w/cpp/language/cv

Some uses of volatile are deprecated: ...

volatile object type as function parameter type or return type;

[ledergec]

This seems to be the only reason which prevent to compile with c++20 standard using clang 10.

[ledergec]

I have create the following PR to fix this:
#1228

[BillyDonahue]

Fixed by #1228