Upgrade to Rails 7.1

[mkllnk]

What we should change and why (this is tech debt)

The next Rails version has been released. There are some performance optimisations which would be nice to have and some new tools we can explore. But there are several steps we need to take first:

• Wait for all our dependencies to be compatible with Rails 7.1.
• Rails 6.1 Deprecation warnings #7853
• Address any other existing deprecation warnings.
• Follow the upgrade guide.

Context

Impact and timeline

Not critical at the moment. But good to chip away at this issue.

[dacook]

Just noting that it would be worth waiting until the Ubuntu server upgrades](openfoodfoundation/ofn-install#157) are done, then we can upgrade Node, then Rails.

[dacook]

I just discovered that our current Node version is blocked until we get all servers onto Ubuntu 20 🤦

But the node version doesn't seem to be a blocker for the Rails upgrade. At least, I didn't find any evidence online.
In a previous CI run (#12492), many of the specs succeeded, so I guess the current node version is fine.

[dacook]

I think all dependencies must be ready for Rails 7.1, because at least some CI runs succeed: https://github.com/dacook/openfoodnetwork/actions/runs/13106684816