[PLT-1390] Removing DPC sns topic key references.

[jscott-nava]

🎫 Ticket

https://jira.cms.gov/browse/PLT-1390

🛠 Changes

This PR removes TF data references to the now deleted DPC SNS topic key, which was removed from the dpc-ops repo in a related PR.

ℹ️ Context

As part of the alarm-to-slack service work the DPC CloudWatch alarm topic was updated to use the shared dpc-<env> KMS key instead of the custom DPC SNS topic key, and that custom key was destroyed. This PR cleans up references to that custom key that continued to exist in the github-actions-role service.

🧪 Validation

Tofu plan output (DPC-DEV)

OpenTofu will perform the following actions:

  # aws_iam_role_policy.github_actions_role_policy will be updated in-place
  ~ resource "aws_iam_role_policy" "github_actions_role_policy" {
        id          = "dpc-dev-github-actions:terraform-2025xxxxxxxxxxxxxxxxxxxxxx"
        name        = "terraform-2025xxxxxxxxxxxxxxxxxxxxxx"
      ~ policy      = jsonencode(
          ~ {
              ~ Statement = [
                    # (12 unchanged elements hidden)
                    {
                        Action   = [
                            "kms:ListAliases",
                            "kms:GetKeyRotationStatus",
                            "kms:GetKeyPolicy",
                            "kms:EnableKeyRotation",
                            "kms:CreateKey",
                            "kms:CreateAlias",
                        ]
                        Effect   = "Allow"
                        Resource = "*"
                        Sid      = "KmsUsage"
                    },
                  ~ {
                      ~ Resource = [
                            # (3 unchanged elements hidden)
                            "arn:aws:kms:us-east-1:xxxxxxxxxxxx:alias/dpc-dev-web-admin-cloudwatch-key",
                          - "arn:aws:kms:us-east-1:xxxxxxxxxxxx:alias/dpc-dev-sns-topic-key",
                            "arn:aws:kms:us-east-1:xxxxxxxxxxxx:alias/dpc-dev-master-key",
                            # (4 unchanged elements hidden)
                        ]
                        # (3 unchanged attributes hidden)
                    },
                    {
                        Action   = [
                            "firehose:StartDeliveryStreamEncryption",
                            "firehose:DescribeDeliveryStream",
                            "firehose:CreateDeliveryStream",
                        ]
                        Effect   = "Allow"
                        Resource = "*"
                    },
                    # (12 unchanged elements hidden)
                ]
                # (1 unchanged attribute hidden)
            }
        )
        # (2 unchanged attributes hidden)
    }

Plan: 0 to add, 1 to change, 0 to destroy.