msvc: tidy up _CRT_*_NO_DEPRECATE definitions

Dedupe and migrate MSVC-specific warning suppressions to `curl_setup.h`.
Make cmake set `_CRT_SECURE_NO_DEPRECATE` for examples and standalone
tests, and stop setting `_CRT_NONSTDC_NO_DEPRECATE` for them.

Details:
- drop version guards. On ancient MSVC version these macro are a no-op.
- move to `curl_setup.h` from `config-win32*.h`.
- sync macro values with CMake.
- cmake: stop setting them globally in favour of `curl_setup.h`.
- cmake: re-add these macros to `docs/examples` and `tests/http/clients`,
  which do not use `curl_setup.h`.
- cmake: drop `_CRT_NONSTDC_NO_DEPRECATE` for examples and tests.
  They build fine without.
- update comments.

Closes #15960

docs/examples/CMakeLists.txt

@@ -33,7 +33,7 @@ foreach(_target IN LISTS check_PROGRAMS)
   add_executable(${_target_name} EXCLUDE_FROM_ALL "${_target}.c")
   add_dependencies(curl-examples ${_target_name})
   target_link_libraries(${_target_name} ${LIB_SELECTED} ${CURL_LIBS})
-  target_compile_definitions(${_target_name} PRIVATE "CURL_NO_OLDIES")
+  target_compile_definitions(${_target_name} PRIVATE "CURL_NO_OLDIES" "$<$<BOOL:MSVC>:_CRT_SECURE_NO_DEPRECATE>")
   if(LIB_SELECTED STREQUAL LIB_STATIC AND WIN32)
     set_property(TARGET ${_target_name} APPEND PROPERTY COMPILE_DEFINITIONS "CURL_STATICLIB")
   endif()