lib: merge `ENABLE_QUIC` C macro into `USE_HTTP3`

lib: merge `ENABLE_QUIC` C macro into `USE_HTTP3`

Before this patch `lib/curl_setup.h` defined these two macros right
next to each other, then the source code used them interchangeably.

After this patch, `USE_HTTP3` guards all HTTP/3 / QUIC features.
(Like `USE_HTTP2` does for HTTP/2.) `ENABLE_QUIC` is no longer used.

This patch doesn't change the way HTTP/3 is enabled via autotools
or CMake. Builders who enabled HTTP/3 manually by defining both of
these macros via `CPPFLAGS` can now delete `-DENABLE_QUIC`.

Closes #13352