Add slack notification for github-plan-and-apply

[ASTRobinson]

A reference to the issue / Description of it

Issue identified by @sukeshreddyg that no slack notification is triggered if the first job fails, only the second job. This PR adds a Slack notification step to the first job.

How does this PR fix the problem?

see above

How has this been tested?

Please describe the tests that you ran and provide instructions to reproduce.

No

Deployment Plan / Instructions

Will this deployment impact the platform and / or services on it?

No

Checklist (check x in [ ] of list items)

• I have performed a self-review of my own code
• All checks have passed
• I have made corresponding changes to the documentation
• Plan and discussed how it should be deployed to PROD (If needed)

Additional comments (if any)

{Please write here}

[github-actions]

Trivy Scan Success

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</details> #### `Checkov Scan` Success
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CTFLint Scan Success

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Setting default tflint config...
Running tflint --init...
Installing "terraform" plugin...
Installed "terraform" (source: github.com/terraform-linters/tflint-ruleset-terraform, version: 0.9.1)
tflint will check the following folders:

Trivy Scan Success

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Trivy will check the following folders: