Puma: v6.4.1 Release

Bugfixes

• DSL#warn_if_in_single_mode - fixup when workers set via CLI ([#3256])
• Fix idle-timeout not working in cluster mode ([#3235], [#3228], [#3282], [#3283])
• Fix worker 0 timing out during phased restart ([#3225], [#2786])
• context_builder.rb - require openssl if verify_mode != 'none' ([#3179])
• Make puma cluster process suitable as PID 1 ([#3255])
• Improve Puma::NullIO consistency with real IO ([#3276])
• extconf.rb - fixup to detect openssl info in Ruby build ([#3271], [#3266])
• MiniSSL.java - set serialVersionUID, fix RaiseException deprecation ([#3270])
• dsl.rb - fix warn_if_in_single_mode when WEB_CONCURRENCY is set ([#3265], [#3264])

Maintenance

• LOTS of test refactoring to make tests more stable and easier to write - thanks to @MSP-Greg!
• Fix bug in tests re: TestPuma::HOST4 ([#3254])
• Dockerfile for minimal repros: use Ruby 3.2, expect bundler installed ([#3245])
• fix define_method calls, use Symbol parameter instead of String ([#3293])

Docs

• README.md - add the puma-acme plugin ([#3301])
• Remove --keep-file-descriptors flag from systemd docs ([#3248])
• Note symlink mechanism in restart documentation for hot restart ([#3298])