Finish v4.0.0 release

[kvz]

Why

v4.0.0 is tagged but not yet published. Before releasing it, the migration guidance needs to describe the Ruby 3.1 requirement accurately, and CI/tooling should verify the SDK against the current supported Ruby range through Ruby 4.0.

What

• Correct the migration guide and changelog for the Ruby 3.0 to 3.1 minimum-version bump.
• Test Ruby 3.1, 3.2, 3.3, 3.4, and 4.0 in CI, with coverage, Node parity, and live e2e verification on Ruby 4.0.
• Update the Docker development image and Ruby 4-sensitive CGI require.
• Require Ruby 4-compatible VCR and Standard versions for development.
• Keep generated Docker and RuboCop caches out of formatter input and Git status.

After merge, the unpublished v4.0.0 tag will be moved to the merge commit before the gem and GitHub release are published.

Verification

• DOCKER_PLATFORM=linux/arm64 TEST_NODE_PARITY=1 RUBY_SDK_E2E=1 ./scripts/test-in-docker.sh (124 runs, 270 assertions, 0 failures, 0 errors, 0 skips)
• DOCKER_PLATFORM=linux/arm64 RUBY_SDK_E2E=0 ./scripts/test-in-docker.sh bundle exec standardrb
• DOCKER_PLATFORM=linux/arm64 RUBY_SDK_E2E=0 ./scripts/test-in-docker.sh gem build transloadit.gemspec

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.40%. Comparing base (043a954) to head (420bb70).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #78   +/-   ##
=======================================
  Coverage   99.40%   99.40%           
=======================================
  Files           9        9           
  Lines         336      336           
  Branches       38       38           
=======================================
  Hits          334      334           
  Misses          1        1           
  Partials        1        1           

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