script/ceph-backport.sh: adjust to change in github API

script/ceph-backport.sh: adjust to change in github API

This curl command was working fine before, but recently it started
throwing the following error:

+ curl --silent --data-binary
'{"milestone":"$milestone_number","assignee":"smithfarm","labels":["bug
fix","core"]}'
'https://api.github.com/repos/ceph/ceph/issues/25018?access_token=8ecba7355a11fab00365e44fece350bf01388553'
{
  "message": "Invalid request.\n\nFor 'properties/milestone',
\"$milestone_number\" is not an integer or null.",
  "documentation_url": "https://developer.github.com/v3"
}

Signed-off-by: Nathan Cutler <ncutler@suse.com>