allow to rename workflows

[migmartri]

renaming workflows should be allowed but we should show the user a warning telling her that the attestation process that points to the previous workflow name must be changed.

[jiparis]

Please keep in mind that currently the workflow name is its ID, because we use it declaratively from the command line. There could be consequences with this action on existing CI integrations.
So we should ensure that we could still support the old one, maybe by supporting name aliases. This is how for example many CMSs work: when a page is renamed, the URL for the previous one still works not to break links.