Frozen literal string deprecation warnings in Ruby 3.4.0-preview2

[jeremyevans]

I found these will running Sequel's test suite with Ruby 3.4.0-preview2:

/usr/local/lib/ruby/gems/3.4/gems/ruby-mysql-4.1.0/lib/mysql/protocol.rb:528: warning: literal string will be frozen in the future
/usr/local/lib/ruby/gems/3.4/gems/ruby-mysql-4.1.0/lib/mysql/protocol.rb:571: warning: literal string will be frozen in the future
/usr/local/lib/ruby/gems/3.4/gems/ruby-mysql-4.1.0/lib/mysql/protocol.rb:795: warning: literal string will be frozen in the future

Replacing the literal strings with String.new calls should fix the issues.