Use GraphQL::Tracing::DataDogTrace when it's available

[rmosolgo]

What does this PR do?

Uses the new GraphQL-Ruby trace_with API directly, instead of via use ...., to avoid a deprecation warning.

Motivation:

Someone reported the deprecation warning to me

Additional Notes:

Although this changes how the module is applied, it doesn't actually change behavior on recent GraphQL-Ruby versions, because GraphQL-Ruby was applying this new module even when the old one was attached: https://github.com/rmosolgo/graphql-ruby/blob/1a0b0dea2f9134c63f20228001c785f9cbaea3d9/lib/graphql/tracing/platform_tracing.rb#L82-L85

So, I think users have already been using the new code.

How to test the change?

Unit tests

For Datadog employees:

• If this PR touches code that signs or publishes builds or packages, or handles
  credentials of any kind, I've requested a review from @DataDog/security-design-and-guidance.
• This PR doesn't touch any of that.

Unsure? Have a question? Request a review!

[TonyCTHsu]

👋 @rmosolgo , I already have this in our 2.0 release. #3417

[aliciascott]

looks good for docs!

[rmosolgo]

That looks great, @TonyCTHsu, thanks!