docs: upgrade warning to error for v3

Author: wdconinc

setup-cvmfs.sh

@@ -34,9 +34,8 @@ if [ "$(uname)" == "Linux" ]; then
   fi
 elif [ "$(uname)" == "Darwin" ]; then
   # Warn about the phasing out of MacOS support for this action
-  echo "::warning::The CernVM-FS GitHub Action's support for MacOS will be \
-phased out with macos-10.15, which will be unsupported by GitHub by 8/30/22. \
-See https://github.com/cvmfs-contrib/github-action-cvmfs/pull/17 for details."
+  echo "::error::The CernVM-FS GitHub Action's support for MacOS has been \
+phased out with macos-10.15."
   # Temporary fix for macOS until cvmfs 2.8 is released
   if [ -z "${CVMFS_HTTP_PROXY}" ]; then
     export CVMFS_HTTP_PROXY='DIRECT'