fix: require test-kitchen 3.0 or newer

[tas50]

The dependency floor was >= 1.0.0:

spec.add_dependency "test-kitchen", ">= 1.0.0", "< 5.0"

That has not been true for a long time. The gem already declares required_ruby_version >= 3.1, Test Kitchen 1.x and 2.x are both end of life, and CI has never run against anything below 4. Bundler was free to resolve a pairing nobody has ever run.

spec.add_dependency "test-kitchen", ">= 3.0", "< 5.0"

The < 5.0 ceiling is unchanged.

rake style clean; rspec 313 examples, 0 failures; bundle check satisfied.

---

One thing worth knowing before this merges. I tested the gem against real Test Kitchen 1.25.0 / 2.12.0 / 3.0.0 / 3.9.1 bundles, and it does not currently work on Test Kitchen 3 at all:

$ kitchen create          # test-kitchen 3.9.1, real ubuntu-24.04
-----> Creating <default-ubuntu-2404>...
>>>>>> Failed to complete #create action:
       [undefined method 'to_blob' for an instance of OpenSSL::PKey::RSA]

generate_keys calls OpenSSL::PKey::RSA#to_blob, which net-ssh adds by reopening the class. The driver requires net-ssh as require "net/ssh" unless defined?(Net::SSH) — but Test Kitchen 3's kitchen/ssh.rb contains:

module Net
  autoload :SSH, "net/ssh"
end

An autoload defines the constant immediately as a placeholder, so defined?(Net::SSH) is truthy before net-ssh has loaded a line. The guard skips the require, the monkey patch is never applied, and every Linux kitchen create dies. Test Kitchen 4 dropped kitchen/ssh.rb, which is the only reason this is invisible today.

That is a separate one-line fix (drop the guard — the require is for a side effect, not a constant), so I have left it out of this PR. Happy to open it next; this floor bump is still strictly an improvement on >= 1.0.0 either way.