Switch macOS minimum version flag to gcc compatible version

Fixes #20698

I'm not sure how that user got into that state yet, but it doesn't hurt for this to be gcc compatible just in case the user is intentionally using gcc on macOS

Closes #20798.

PiperOrigin-RevId: 604515755
Change-Id: Ib9087fe6afbbb3514513a7d5d120fcf448e54f65

tools/cpp/unix_cc_toolchain_config.bzl

@@ -1337,7 +1337,7 @@ def _impl(ctx):
         flag_sets = [
             flag_set(
                 actions = all_compile_actions + all_link_actions,
-                flag_groups = [flag_group(flags = ["-mmacos-version-min={}".format(_target_os_version(ctx))])],
+                flag_groups = [flag_group(flags = ["-mmacosx-version-min={}".format(_target_os_version(ctx))])],
             ),
         ],
     )