Dependabot alerts issue assumes that CMS 5 is the most recent version

`.github/workflows/dependabot-prs-issue.yml` has `5` hardcoded as the current version of the CMS

Instead this should be looping through all currently supported CMS versions (5 and 6 right now) and linking to all those repositories, in case any new ones were added e.g. https://github.com/silverstripe/silverstripe-htmleditor-tinymce

