ci-fairy: switch to project-specific linters
The old instance-wide lint endpoint no longer exists in gitlab 16.0 and later, so let's do this project-wide.
Since our only use-case here is to upload a file and validate it, the actual project ID can default to the ci-templates one - after all the YAML file should be valid for any project. This will not work if the CI configuration has project-specific includes directives - those will the project be specified (or some future code to guess which project we're dealing with).
Fixes #60 (closed)
Future code that could be interesting: check the origin remote and extract the project name from that. But I think for most use-cases (if anyone even uses linting, because we need an active token, see #33 (closed)) this here will be good enough.