[4.0] add bigdecimal to gemspec for ruby 3.4

[mathbruyen]

bigdecimal was removed from stdlib in Ruby 3.4 and liquid fails to be loaded:

0.679 /usr/local/bundle/gems/liquid-4.0.4/lib/liquid/standardfilters.rb:2:in 'Kernel#require': cannot load such file -- bigdecimal (LoadError)                                                                     
0.679   from /usr/local/bundle/gems/liquid-4.0.4/lib/liquid/standardfilters.rb:2:in '<top (required)>'
0.679   from /usr/local/bundle/gems/liquid-4.0.4/lib/liquid.rb:72:in 'Kernel#require'
0.679   from /usr/local/bundle/gems/liquid-4.0.4/lib/liquid.rb:72:in '<top (required)>'
0.679   from /usr/local/bundle/gems/jekyll-4.3.4/lib/jekyll.rb:35:in 'Kernel#require'

Fixes #1772 which also mentions base64, but the later is not directly used by liquid thus not added.

Similar pull request but targeted at main which already has the change (5.x releases have the depenency), can likely be closed: #1779

[ggmichaelgo]

Hello @mathbruyen 👋

Since the Liquid version 4 releases are 3+ years old, we should try to use the latest version of Liquid.

It looks like there is already PR to update Jekyll's Liquid version:
jekyll/jekyll#9722