[usockets,uwebsockets] Update uwebsockets to 20.71.0, install and test CMake config, Cleanup

[dg0yt]

Install unofficial CMake config. Resolves #42239.

uwebsockets: Update.

• Move zlib to optional feature.
• Wire ssl support.

usockets: Cleanup. Align CMakeLists.txt with upstream Makefile.

• Support only the default event loop for the target platform as in
  https://github.com/uNetworking/uSockets/blob/8606de6414a102c55bef8e8ef3391932d7e8df6a/src/libusockets.h#L339-L348
  i.e. libuv is used on Windows only.
• Remove the network feature with its unique winsock2 dependency.
  usockets was the last port modeling the winsock2 dependency. After merge of this PR, winsock2 should be delisted. (Another PR.)

[dg0yt]

Ping @FrankXie05.