drop support for Ruby 3.0, add support for Ruby 3.4

[flavorjones]

Ruby 3.0 was EOL 23 Apr 2024. Nokogiri's policy is to support the current and previous three versions of Ruby, so we won't drop support until Ruby 3.4 is released.

Tagging this for v1.18.0 which will coincide with Ruby 3.4.0 final release in Dec 20242.

This will likely happen in v1.18 release candidates earlier than December.

• drop 3.0 from cross-rubies and CI and docker.rake
• add 3.4 to cross-rubies and docker.rake
• delete 3.0 valgrind suppressions
• remove windows MSVCRT builds
• update gemspec required_ruby_version to ">= 3.1.0"
• update README
• bump rubocop TargetRubyVersion to "3.1"
• remove any tests or code relevant only to Ruby 3.0
• remove unnecessary extconf.rb compile-time checks