Update Contributor metadata

We support a Contribution Annotation Framework that allows us to indicate roles of conributors. However, many tutorials still use the "old" style contribution metadata (without roles)

Updating this involves rewriting old-style `contributors` metadata to new-style `contributions` metadata (see [FAQ](https://training.galaxyproject.org/training-material/topics/contributing/faqs/new-contributions.html)), with any contributors being given the author role.


Depending on your skillset, this could be done manually, or via a script to do a bulk find and replace.

Start with the linked FAQ to understand the new Contribution Annotation Framework, then search the repository for old-style `contributors` metadata. Rewrite each occurrence using `contributions` with the author role, and verify that no old-style metadata remains and the tutorials still render correctly.

Written by the indexing model from the issue text.