[v3.2] Notify CI failures on Slack

[waiting-for-dev]

Description

Backports #284 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.

Motivation and Context

We want to get an alert when a build on a supported branch is broken.

How Has This Been Tested?

Manually on a fork and a private slack app.

Screenshots (if appropriate):

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [ ] I have updated the documentation accordingly.