Bigdecimal won't be part of standard library from Ruby 3.4.0 #97

[SandeepTuniki]

(Duplicating this issue from googleapis/ruby-spanner#97, since we have multiple gems in this repo using bigdecimal)

Saw this warning log message during a CI run:

/[workspace/google-cloud-spanner/lib/google/cloud/spanner/convert.rb:20]
warning: bigdecimal was loaded from the standard library,
but will no longer be part of the default gems since Ruby 3.4.0.
Add bigdecimal to your Gemfile or gemspec.

A quick project-wide search shows the following gems are using bigdecimal:

• bigquery
• firestore
• datastore
• pubsub

Creating this bug to keep track of the warning before it becomes an error in the future.