build: enable (non-fatal) -Wsign-conversion warnings (OpenSSF)

Warn by default in all builds, but without triggering error.

Once all new warnings revealed by CI are cleared, we can drop the
`-Wno-error=sign-conversion` options in cmake/autotools.

Follow-up to3829759bd042c03225ae862062560f568ba1a231 curl#12489
Closes #xxxxx

.github/workflows/linux.yml

@@ -111,7 +111,7 @@ jobs:
           - name: openssl3-O3
             install_packages: zlib1g-dev valgrind
             install_steps: gcc-11 openssl3
-            configure: CPPFLAGS=-DCURL_WARN_SIGN_CONVERSION CFLAGS=-O3 LDFLAGS="-Wl,-rpath,$HOME/openssl3/lib" --with-openssl=$HOME/openssl3 --enable-debug --enable-websockets
+            configure: CFLAGS=-O3 LDFLAGS="-Wl,-rpath,$HOME/openssl3/lib" --with-openssl=$HOME/openssl3 --enable-debug --enable-websockets
             singleuse: --unit
 
           - name: openssl3-clang

lib/curl_setup.h

@@ -28,13 +28,6 @@
 #define CURL_NO_OLDIES
 #endif
 
-/* FIXME: Delete this once the warnings have been fixed. */
-#if !defined(CURL_WARN_SIGN_CONVERSION)
-#ifdef __GNUC__
-#pragma GCC diagnostic ignored "-Wsign-conversion"
-#endif
-#endif
-
 /* Set default _WIN32_WINNT */
 #ifdef __MINGW32__
 #include <_mingw.h>