windows: drop code and curl manifest targeting W2K and older

curl requires Windows XP since 2023. Drop version detection code using
`GetVersionEx()` aimed to support earlier Windows versions. With that
call deleted, the embedded manifest in `curl.rc` becomes unnecessary.
Delete it too, along with the enabler logic in build systems.

This allows to stop forcing `/MANIFEST:NO` for MSVC builds. Dropping it
fixes VS2008 shared builds, that require an auto-generated SxS
(side-by-side assembly) manifest to find their CRT DLLs. This was the
issue that prevented VS2008 `curl.exe` launching on AppVeyor CI:
```
src/curl.exe: error while loading shared libraries: ?: cannot open shared object file: No such file or directory
```
Ref: https://ci.appveyor.com/project/curlorg/curl/builds/51577006/job/eitypvwlb1rxr11d#L261

FWIW the `curl.rc` embedded manifest wasn't ever enabled for VS2008 CI
builds either, because CMake did not pass our custom macro via
`CMAKE_RC_FLAGS` to `rc.exe`. For reasons I could not figure out.

After this patch the curl build no longer inject its own manifest, and
lets the default be applied by linkers and toolchains. It fixes VS2008
shared builds. curl continues to detect the real Windows version via
`RtlVerifyVersionInfo()` from `ntdll`.

Follow-up to 960d601481 #12225
Follow-up to 5044909ca2 #7810
Follow-up to ebd213270a #1221
Ref: #15972
Cherry-picked from #16394

Closes #16453

CMakeLists.txt

@@ -1983,9 +1983,6 @@ if(WIN32)
     set(USE_WIN32_LARGE_FILES ON)
   endif()
 
-  # Use the manifest embedded in the Windows Resource
-  string(APPEND CMAKE_RC_FLAGS " -DCURL_EMBED_MANIFEST")
-
   # We use crypto functions that are not available for UWP apps
   if(NOT WINDOWS_STORE)
     set(USE_WIN32_CRYPTO ON)
@@ -2001,9 +1998,6 @@ if(WIN32)
 endif()
 
 if(MSVC)
-  # Disable default manifest added by CMake
-  string(APPEND CMAKE_EXE_LINKER_FLAGS " -MANIFEST:NO")
-
   string(APPEND CMAKE_C_FLAGS " -MP")  # Parallel compilation
 endif()