[v3.2] Notify CI failures on Slack

[waiting-for-dev]

Summary

Backports #24 to v3.2

We use the circleci/slack orb to notify whenever a CircleCI job fails. We only track builds on main and branches tracking old Solidus releases. All jobs are included.

Currently, the CircleCI context has been configured to point to the #ci-notifications channel on [Solidus' Slack workspace (https://solidusio.slack.com), where we have created the required Slack app.

Checklist

Check out our PR guidelines for more details.

The following are mandatory for all PRs:

• [x] I have written a thorough PR description.
• [x] I have kept my commits small and atomic.
• [x] I have used clear, explanatory commit messages.

The following are not always needed (~cross them out~ if they are not):

• ~[ ] I have added automated tests to cover my changes.~
• [x] I have attached screenshots to demo visual changes.
• ~[ ] I have opened a PR to update the guides.~
• ~[ ] I have updated the README to account for my changes.~