gem: add bigdecimal as dependency for Ruby 3.4

[Watson1978]

This patch will fix following error:

/home/runner/.rubies/ruby-head/lib/ruby/4.1.0+1/bundled_gems.rb:60:in 'Kernel.require': cannot load such file -- bigdecimal (LoadError)
Did you mean?  bson/big_decimal
	from /home/runner/.rubies/ruby-head/lib/ruby/4.1.0+1/bundled_gems.rb:60:in 'block (2 levels) in Kernel#replace_require'
	from /home/runner/.rubies/ruby-head/lib/ruby/gems/4.1.0+1/gems/bson-4.15.0/lib/bson/decimal128.rb:16:in '<top (required)>'
	from /home/runner/.rubies/ruby-head/lib/ruby/4.1.0+1/bundled_gems.rb:60:in 'Kernel.require'
	from /home/runner/.rubies/ruby-head/lib/ruby/4.1.0+1/bundled_gems.rb:60:in 'block (2 levels) in Kernel#replace_require'
	from /home/runner/.rubies/ruby-head/lib/ruby/gems/4.1.0+1/gems/bson-4.15.0/lib/bson.rb:76:in '<top (required)>'
	from /home/runner/.rubies/ruby-head/lib/ruby/4.1.0+1/bundled_gems.rb:60:in 'Kernel.require'
	from /home/runner/.rubies/ruby-head/lib/ruby/4.1.0+1/bundled_gems.rb:60:in 'block (2 levels) in Kernel#replace_require'
	from /home/runner/.rubies/ruby-head/lib/ruby/gems/4.1.0+1/gems/mongo-2.18.3/lib/mongo.rb:35:in '<top (required)>'
	from /home/runner/.rubies/ruby-head/lib/ruby/4.1.0+1/bundled_gems.rb:60:in 'Kernel.require'
	from /home/runner/.rubies/ruby-head/lib/ruby/4.1.0+1/bundled_gems.rb:60:in 'block (2 levels) in Kernel#replace_require'
	from /home/runner/work/fluent-plugin-mongo/fluent-plugin-mongo/test/helper.rb:3:in '<top (required)>'
	from /home/runner/.rubies/ruby-head/lib/ruby/4.1.0+1/bundled_gems.rb:60:in 'Kernel.require'
	from /home/runner/.rubies/ruby-head/lib/ruby/4.1.0+1/bundled_gems.rb:60:in 'block (2 levels) in Kernel#replace_require'
	from /home/runner/work/fluent-plugin-mongo/fluent-plugin-mongo/test/plugin/test_in_mongo_tail.rb:1:in '<top (required)>'
	from /home/runner/.rubies/ruby-head/lib/ruby/4.1.0+1/bundled_gems.rb:60:in 'Kernel.require'
	from /home/runner/.rubies/ruby-head/lib/ruby/4.1.0+1/bundled_gems.rb:60:in 'block (2 levels) in Kernel#replace_require'
	from /home/runner/.rubies/ruby-head/lib/ruby/gems/4.1.0+1/gems/rake-13.3.1/lib/rake/rake_test_loader.rb:21:in 'block in <main>'
	from /home/runner/.rubies/ruby-head/lib/ruby/gems/4.1.0+1/gems/rake-13.3.1/lib/rake/rake_test_loader.rb:6:in 'Array#select'
	from /home/runner/.rubies/ruby-head/lib/ruby/gems/4.1.0+1/gems/rake-13.3.1/lib/rake/rake_test_loader.rb:6:in '<main>'
rake aborted!

[kenhys]

LGTM.