gnu: ruby: Switch from 2.7 to 3.1.

I did look at switching to 3.2, but Rails still needs 3.1 I think, so 3.2 will
be possible once Rails 7.1 is released.

* gnu/packages/ruby.scm (ruby): Switch from 2.7 to 3.1.

gnu/packages/ruby.scm

@@ -261,7 +261,7 @@ a focus on simplicity and productivity.")
      (modify-inputs (package-inputs ruby-3.1)
        (prepend libyaml)))))
 
-(define-public ruby ruby-2.7)
+(define-public ruby ruby-3.1)
 
 (define-public mruby
   (package