Update Rails

[Flaburgan]

Rails 7 is now out, and Rails 5 doesn't officially support Ruby 2.7 (see rails/rails#38426) so we need to upgrade, probably to Rails 6 first.

[ragesoss]

It looks like this will depend on:

1. the upcoming release of version 0.3.0 of https://github.com/diaspora/diaspora_federation
2. an update to — or replacement of — js_image_paths. Since there were some asset pipeline changes from Rails 5 to Rails 6, updating js_image_paths might require more than just relaxing the Rails version constraint.

[tclaus]

• The change to Zeitwerk - class loader will be challenging.
• Some Gems are quite old or not directly supported.
• Workers now based on ActiveJob (easy).

[abrahamparayil]

We could mark this issue as an Epic and create sub-issues for the individual tasks that needs to be completed before we can upgrade to rails 6. That way we can make this easier to track and hopefully make it easier for more people to contribute to the effort.