shields.io badges will fail on private repo

This is what we get on private repositories on a successful backport, for the link to the backport.

The code here https://github.com/elastic/kibana-github-actions/blob/main/backport/createStatusComment.ts#L24
is using `https://img.shields.io/github/pulls/detail/state/${repoOwner}/${repoName}/${backportPullNumber}`

which won't work on any private repo.

It looks like it's quite complicated to do properly (see https://github.com/badges/shields/issues/593)
my proposal is to fallback to a generic image. We could do this in full JS on the client side when the image 404s..