Dalli 4.0.0 release

[petergoldstein]

Summary

This PR prepares the Dalli 4.0.0 release with breaking changes, security improvements, and expanded test coverage.

Breaking Changes

• Ruby 3.1+ required - Dropped support for Ruby 2.6, 2.7, and 3.0
• Removed Dalli::Server - Use Dalli::Protocol::Binary instead
• Removed :compression option - Use :compress instead
• Removed close_on_fork method - Use reconnect_on_fork instead

New Features & Improvements

• Add security warning when using default Marshal serializer (can be silenced with silence_marshal_warning: true)
• Add defense-in-depth input validation for stats command arguments
• Add string_fastpath option to skip serialization for simple strings (byroot)
• Meta protocol set performance improvement (danmayer)
• Fix connection_pool 3.0 compatibility for Rack session store

Test Coverage

• Add unit tests for binary protocol response processor
• Add unit tests for meta protocol response processor
• Add unit tests for meta protocol key regularizer
• Add tests for stats input validation
• Add tests for Marshal warning behavior

CI Updates

• Update memcached test version from 1.6.34 to 1.6.40
• Remove Ruby 2.6, 2.7, 3.0 from test matrix
• Apply Ruby 3.1+ syntax improvements (anonymous block forwarding, Hash#except)

Test plan

• All existing tests pass (535 runs, 143,385 assertions, 0 failures)
• Rubocop passes with no offenses
• New unit tests cover response processors and key regularizer
• CI matrix tests Ruby 3.1, 3.2, 3.3, 3.4, 4.0, head, jruby-10
• CI matrix tests memcached 1.5.22 and 1.6.40

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