fix: Point Shared Workflows to main

[milldr]

what

• Update workflow action references to use main branch

why

• The previously mentioned branch does not exist anymore. It was merged with [ci-readme] Remove unused inputs github-actions-workflows#117

references

• Example of failing module: https://github.com/cloudposse/terraform-aws-managed-prometheus/actions/runs/9292957421

[Invalid workflow file: .github/workflows/branch.yml#L24](https://github.com/cloudposse/terraform-aws-managed-prometheus/actions/runs/9292957421/workflow)
error parsing called workflow
".github/workflows/branch.yml"
-> "cloudposse/.github/.github/workflows/shared-terraform-module.yml@main" (source branch with sha:a7920175625a9e55449656baafda1bd2a82603ba)
--> "cloudposse/github-actions-workflows/.github/workflows/ci-terraform.yml@shared-workflows"
: failed to fetch workflow: reference to workflow should be either a valid branch, tag, or commit

[mergify]

Important

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