✨ Add Slack Notification on Terraform Apply Failure

[connormaglynn]

👀 Purpose

• In relation to #975
• To make apply failures when code is merged into main more visible

♻️ What's changed

• Added a Slack alert to both terraform apply workflows when the workflow fails on main
• Added a temporary test workflow on PR and Push to main - just so I can make sure everything looks okay before closing off the ticket 🙈

📝 Notes

• A new Slack app has been added for this feature, which all the @ministryofjustice/aws-root-account-admin-team have been added to
• While I'm testing the syntax of the alert I've added a test workflow and webhook, which is going to #connors-testing-channel
• Some information isn't present on the Pull Request event that (supposably) is available of the push event 🙈 Which is why I've got the temporary test workflow - which will be deleted after I've confirmed all looks good on the push event 🔥

[github-actions[bot]]

🦙 MegaLinter status: ✅ SUCCESS

Descriptor	Linter	Files	Fixed	Errors	Elapsed time
✅ ACTION	actionlint	3		0	0.04s
✅ REPOSITORY	gitleaks	yes		no	0.74s
✅ REPOSITORY	trivy	yes		no	9.48s
✅ YAML	prettier	3		0	0.55s

See detailed report in MegaLinter reports _Set VALIDATE_ALL_CODEBASE: true in mega-linter.yml to validate all sources, not only the diff_

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