Update dependabot.yml

[devoncarew]

• use type-infra labels for dependabot PRs

---

• I’ve reviewed the contributor guide and applied the relevant portions to this PR.

Contribution guidelines:

• See our contributor guide for general expectations for PRs.
• Larger or significant changes should be discussed in an issue before creating a PR.
• Contributions to our repos should follow the Dart style guide and use dart format.
• Most changes should add an entry to the changelog and may need to rev the pubspec package version.
• Changes to packages require corresponding tests.

Note that many Dart repos have a weekly cadence for reviewing PRs - please allow for some latency before initial review feedback.

[copybara-service]

Thank you for your contribution! This project uses Gerrit for code reviews. Your pull request has automatically been converted into a code review at:

https://dart-review.googlesource.com/c/homebrew-dart/+/318380

Please wait for a developer to review your code review at the above link; you can speed up the review if you sign into Gerrit and manually add a reviewer that has recently worked on the relevant code. See CONTRIBUTING.md to learn how to upload changes to Gerrit directly.

Additional commits pushed to this PR will update both the PR and the corresponding Gerrit CL. After the review is complete on the CL, your reviewer will merge the CL (automatically closing this PR).