Specify maximum compatible Ruby version for Rails 5

[chris-hewitt]

This is important to note because the section goes on to say "Upgrade to the latest Ruby you can first, and then upgrade Rails."

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[eileencodes]

I think we should just remove this line instead of updating it, Rails 5 is no longer supported.

[chris-hewitt]

This is the document that describes how to move to a supported version. See https://guides.rubyonrails.org/upgrading_ruby_on_rails.html#upgrading-from-rails-5-2-to-rails-6-0

[matthewd]

cc #47811, including some consideration of ways to maintain this info outside the main repo, to side step the problem that the max Ruby version is only really defined after the Rails version ceases to be supported.

(It's also not quite this easy to define the max version: all of Rails 5.* is compatible with Ruby 2.2, but I doubt all of those work with Ruby 2.6.)

[chris-hewitt]

Agreed. I think somehow, though, the guide should describe what steps to take and in what order. Sections 1.2 and 1.3 almost get us there, except (currently) for the case of upgrading to Rails 6. In that case the "latest Ruby you can" instruction needs a caveat, so users don't waste time upgrading to Ruby 2.7/3.0/etc, only to get a Rails error.

[zzak]

This is not necessarily a comment on this PR specifically, but I think that upgrading guide could be split into multiple separate pages, per upgrade path, with the base page talking about overview concepts and links to all of the guides IMO.

[p8]

A duplicate of this PR got merged.
Not sure this can be closed, as there is still some discussion going on.

[zzak]

Whoops, yeah I saw the other PR and forgot, it's been reverted. 🙇