Update fetch-upstream

scripts/fetch-upstream

 #!/bin/bash
 
 REMOTE=upstream
-REMOTE_URL=https://github.com/Icinga/deb-icinga2.git
+REMOTE_URL=https://git.icinga.com/packaging/deb-icinga2.git
 
 if ! git config "remote.${REMOTE}.url" &>/dev/null; then
   git remote add "${REMOTE}" "${REMOTE_URL}"