• Update workflow to use v4 tag.

The next branch on this repo is several months old and was last updated by @Uzlopak. We really need to be coordinating with the "I'm interested in working on this" issue prior to starting pull requests so that we know we have a valid branch to target. As it is, I have created https://github.com/fastify/avvio/tree/next-fastify-v5 for now and rebased this PR. Some coordination needs to happed to determine if this is appropriate or if changes in the current next branch need to be incorporated.

Also, a word of warning: this is a complicated module. We are likely going to want @mcollina's input on this one.

This PR now contains the updates I'm aware of being needed from fastify/fastify#5116. If something additional needs to happen (as either this PR or a separate one) we can discuss it either here or in #245.

I think we can combine them in next

If I understand correctly, @Uzlopak just removed express; there is no big change underway

Edit: asked Uzlopak directly