Add Ruby 4.0 support

[tas50]

Problem

require "serverspec" fails on Ruby 4.0:

LoadError: cannot load such file -- logger
    from lib/serverspec.rb:4:in '<top (required)>'

net-ssh (pulled in via specinfra) does require 'logger' on its first line but does not declare the dependency. That was fine while logger was a default gem — always on the load path. As of Ruby 3.5 / 4.0 it is a bundled gem, so Bundler no longer puts it on the load path unless something in the dependency graph asks for it.

This is the same class of fix specinfra already made for base64 during the Ruby 3.4 cycle.

Changes

• Declare logger as a runtime dependency in the gemspec.
• Add "4.0" to the CI matrix.

Compatibility with the existing matrix

The unconditional dependency resolves on every Ruby currently tested, because the oldest logger releases declare no Ruby floor at all:

Ruby	resolves
2.2	logger 1.3.0 (required_ruby_version: >= 0)
2.3	logger 1.5.3
2.5 – 4.0	logger 1.7.0

So the Ruby 2.2 floor is unaffected.

Verification

Full suite on Ruby 4.0.6:

650 examples, 0 failures

Before this change the same suite reported 6 errors occurred outside of examples, all cannot load such file -- logger.

[mizzy]

Thanks for tracking this down — the diagnosis is right, but the fix belongs in specinfra.

The require "logger" comes from specinfra/backend/ssh.rb → net/ssh.rb; serverspec's own lib never references logger. The base64 precedent you cite was mizzy/specinfra#755, against specinfra.gemspec. Declaring it here would also leave itamae (the other specinfra consumer) broken.

Note the CI matrix hunk can't be split out and merged on its own — ci (4.0) only passes here because the gemspec change pulls logger into the bundle. So this needs to land in specinfra first, then serverspec bumps its specinfra floor and adds 4.0 to the matrix.

One thing to watch in the specinfra PR: an unconditional logger dependency breaks Ruby 2.2 (ci (2.2) fails on this branch, master is green). logger 1.3.0 declares no required_ruby_version but uses &. internally, so it shadows the stdlib logger and dies with a SyntaxError. specinfra may just want to drop 2.2 — it's EOL since 2018 and ships in no currently-supported distro — but that's a call for that repo.

Closing here; would you be up for opening it against mizzy/specinfra?