text: Title: Curio still mentioned in docs although unsupported since 0.21.0
  Body: See discussion https://github.com/encode/httpx/discussions/2232?converting=1, see also comment https://github.com/encode/httpx/discussions/1953#discussioncomment-1715439
  
  Curio is still mentioned in documentation here as being supported: https://www.python-httpx.org/async/#curio
  
  But it was dropped in version 0.21.0, when HTTPCore dropped curio support as part of the 0.14.0 redesign.
  
  There is discussion on getting curio support back: https://github.com/encode/httpx/discussions/1953
  
  But right now the documentation is incorrect and curio should be dropped from it.