Remove deprecated (and to be removed) std::allocator<void>

[tobiasneumann]

Bug report

Required Info:

• Operating System:
  • Debian 12
• Installation type:
  • source
• Version or commit hash:
  • HEAD
• DDS implementation:
  • N/A
• Client library (if applicable):
  • N/A

Steps to reproduce issue

Build this code with C++17 or higher and -Wdeprecated-declarations.

Expected behavior

It compiles without warnings.

Actual behavior

It compiles with warnings.

Additional information

std::allocator<void> is deprecated in C++17 and removed in C++20. https://en.cppreference.com/w/cpp/memory/allocator

[clalancette]

Yeah, I see what you mean about the removal. Do you happen to know what the replacement should be? Also pinging @azeey , @wjwwood for advice.