Prepare for resque 3.0 compatibility

[PatrickTulskie]

This PR prepares resque-scheduler for compatibility with the upcoming resque 3.0 release.

Breaking Changes

• Ruby requirement: Updated from >= 2.3.0 to >= 3.0.0 to align with resque 3.0
• Resque dependency: Updated from >= 1.27 to >= 3.0

Dependency Updates

• Updated redis dependency from >= 3.3 to >= 4.0
• Added base64 ~> 0.1 runtime dependency for Ruby 3.4+ compatibility
• Updated rubocop from ~> 0.40.0 to ~> 0.80
• Removed rack < 3 constraint to support both Rack 2 and 3

CI/Testing Improvements

• Updated CI to test Ruby 3.0, 3.1, 3.2, 3.3, 3.4, and head (removed Ruby 2.x)
• Added Rack 2 and 3 matrix testing
• Simplified CI matrix to focus on resque master branch
• Updated rubocop workflow to use Ruby 3.4

Configuration

• Modernized .rubocop.yml to use DisabledByDefault: true approach while preserving existing rules
• Set TargetRubyVersion: 3.0

Note

This PR does not include a version bump to 5.0.0 yet. That will be done when resque 3.0 is actually released.

[PatrickTulskie]

Flipping this over to draft for now. I'll mark it ready for review once resque 3.0 is shipped.