Bump backoff from 0.3.0 to 0.4.0 in /src/agent, resolve dependabot block

[hayleycall]

Summary of the Pull Request

Resolves dependabot PR 1526. This PR updates backoff version from 0.3.0 to 0.4.0 and updates new fields for Backoff::Error::Transient.

PR Checklist

• Applies to work item: #xxx
• CLA signed. If not, go over here and sign the CLI.
• Tests added/passed
• Requires documentation to be updated
• I've discussed this with core contributors already. If not checked, I'm ready to accept this work might be rejected in favor of a different grand plan. Issue number where discussion took place: #xxx

Info on Pull Request

Update Backoff version and change uses of Backoff::Error::Transient.

Validation Steps Performed