Print a release notes link instead of the whole changelog on gem update --system

[hsbt]

gem update --system prints every CHANGELOG.md entry newer than the previous version, which is 478 of the 499 output lines on the ruby:3.4 Docker image. The changelog is generated from merged PR titles, so what gets printed is closer to a PR list than release notes. This replaces the dump with a single line linking the versioned CHANGELOG.md. setup.rb keeps accepting --previous-version so the message can name the range to read. The changelog is printed by the new version's setup.rb child process, so this also takes effect when upgrading from older RubyGems.

With this change, gem update --system prints the link where the changelog dump used to be:

$ gem update --system
Installing RubyGems 4.0.19
...
RubyGems 4.0.19 installed
Regenerating binstubs
Regenerating plugins

See https://github.com/ruby/rubygems/blob/v4.0.19/CHANGELOG.md for the changes since 3.6.9.

------------------------------------------------------------------------------

RubyGems installed the following executables:
	/usr/local/bin/gem
	/usr/local/bin/bundle
	/usr/local/bin/bundler

RubyGems system software updated

The update suggestion shown before updating gets the same link and now matches npm's three-line notice:

A new release of RubyGems is available: 3.6.9 → 4.0.19!
See https://github.com/ruby/rubygems/blob/v4.0.19/CHANGELOG.md for the changes since 3.6.9.
Run `gem update --system 4.0.19` to update your installation.

Fixes #9789

[hsbt]

@kenchan How about this?

[kenchan]

Thanks, this is exactly what I had in mind — really glad to see it.

I went through it with Claude Code and found two cases where the new message isn't accurate. Both only affect ruby setup.rb, not gem update --system. Just sharing them here:

1. On master Gem::VERSION is 4.1.0.dev, so the link comes out as blob/v4.1.0.dev/CHANGELOG.md — a 404, since there's no such tag.
2. Without --previous-version, these lines synthesize one, so the message says "since 4.0.0" from a tarball and "since 4.1.0.dev" from a checkout — either way a version the user never had.