Deprecate Ruby 2.5

[duncanista]

What does this PR do?

Deprecates the runtime Ruby 2.5 for the layer.

Motivation

AWS Lambda ended Phase 2 support earlier this march. Learn more about it here.

Testing Guidelines

Updated integration tests.

Additional Notes

• Pre PR to bump dd-trace from 0.x to 1.x

Types of changes

• [ ] Bug fix
• [ ] New feature
• [ ] Breaking change
• [ ] Misc (docs, refactoring, dependency upgrade, etc.)

Check all that apply

• [ ] This PR's description is comprehensive
• [ ] This PR contains breaking changes that are documented in the description
• [ ] This PR introduces new APIs or parameters that are documented and unlikely to change in the foreseeable future
• [ ] This PR impacts documentation, and it has been updated (or a ticket has been logged)
• [ ] This PR's changes are covered by the automated tests
• [ ] This PR collects user input/sensitive content into Datadog