rename "target" to "artifact"

[jku]

The issue is that TUF (spec and implementation) overloads the word target:

• "targets" is a metadata type
• "target file" is an artifact

This is especially an issue in UI and code where target files are being handled. I think consistently calling them artifacts would alleviate the problem. This is also in-line with what rstuf (another repository implementation) does.

[lukpueh]

I support this. And the same suggestion has been made on the TUF spec issue tracker: https://github.com/theupdateframework/specification/issues/149#issuecomment-878436226

"artifact" is definitely the right word for this thing these days.

[jku]

moved to tuf-on-ci