Remove support for taint checking

[dylanahsmith]

Problem

Running the tests with on ruby 2.7 results in a lot of deprecation warnings like

.../lib/liquid/standardfilters.rb:44: warning: Object#untaint is deprecated and will be removed in Ruby 3.2.

which are from ruby deprecating taint checking.

I was also seeing multiple of the following deprecation warning when running the tests

.../.gem/ruby/2.7.1/gems/rake-11.3.0/lib/rake/application.rb:378: warning: deprecated Object#=~ is called on Proc; it always returns nil

Solution

Remove support for taint checking and upgrade the development version of rake.