Lock bigdecimal version to 3.1.4

This commit addresses the Rails CI using Ruby master branch failure
because Rails CI enables `RAILS_STRICT_WARNINGS` to raise RuntimeError for warnings.

https://buildkite.com/rails/rails/builds/102621#018c49df-e3de-4c2f-aeb7-6d8f08997da9/1102-1107
```ruby
/rails/activesupport/lib/active_support/core_ext/big_decimal/conversions.rb:4: warning: bigdecimal/util is found in bigdecimal, which will no longer be part of the default gems since Ruby 3.4.0. Add bigdecimal to your Gemfile or gemspec. (RuntimeError)
```

According to https://bugs.ruby-lang.org/issues/20058 ,
This failure has been triggered since ruby/ruby@cc98265
and it will be addressed once the bigdecimal 3.1.5 is available at rubygems.org .

In the meantime, we can workaround this issue by logking `bigdecimal` version to 3.1.4 or lower.
This commit should be reverted when the `bigdecimal` 3.1.5 is available at rubygems.org .

Gemfile.lock

@@ -86,7 +86,7 @@ PATH
       marcel (~> 1.0)
     activesupport (7.2.0.alpha)
       base64
-      bigdecimal
+      bigdecimal (< 3.1.5)
       concurrent-ruby (~> 1.0, >= 1.0.2)
       connection_pool (>= 2.2.5)
       drb

activesupport/activesupport.gemspec

@@ -41,5 +41,5 @@ Gem::Specification.new do |s|
   s.add_dependency "minitest",        ">= 5.1"
   s.add_dependency "base64"
   s.add_dependency "drb"
-  s.add_dependency "bigdecimal"
+  s.add_dependency "bigdecimal",      "< 3.1.5"
 end