BigDecimal error on newer ruby

[taggart]

When doing amber rebuild (amber 0.3.12) on my Debian bullseye system (ruby2.7) I am getting the following error:

ERROR: undefined method 'new' for BigDecimal:Class

I think that method was deprecated
https://github.com/ruby/ruby/blob/v2_6_0/NEWS#label-Stdlib+updates+-28outstanding+ones+only-29

https://github.com/ruby/bigdecimal#which-version-should-you-select

It is being used in lib/amber/page_array.rb. The error seems to be triggered by this haml

= child_summaries :include_toc => true, :order_by => [:name, {:direction => :desc, :numeric => true}]

and rather than the summaries the output is "true". This can currently be seen here.

[taggart]

This is still breaking the newsletter archives and would be good to fix.