Update CONTRIBUTING.md

[mit-mit]

Update link to depot tools. The current link (https://www.chromium.org/developers/how-tos/depottools/#installing) now just redirects to here.

• Thanks for your contribution! Please replace this text with a description of what this PR is changing or adding and why, list any relevant issues, and review the contribution guidelines below.

---

• 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.

Note that this repository uses Gerrit for code reviews. Your pull request will be automatically converted into a Gerrit CL and a link to the CL written into this PR. The review will happen on Gerrit but you can also push additional commits to this PR to update the code review.

[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/sdk/+/337161

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).

[copybara-service]

https://dart-review.googlesource.com/c/sdk/+/337161 has been updated with the latest commits from this pull request.