CCM-16644: Update Digital Letters Reporting Rule Policy

[gareth-allan]

Description

This PR updates the IAM policy used by the EventBridge rule that forwards Digital Letters events to the reporting domain, so that it uses the correct name for the KMS key created in this PR in the reporting repo.

Context

This is required because the name of the key actually created differs from the one predicted when we originally created this rule.

Validation

Deployed the changes from this branch, with the changes from this nhs-notify-internal PR to my gaal8 eventbus environment: https://github.com/NHSDigital/nhs-notify-internal/actions/runs/25679915060

I also deployed the changes from this reporting PR to the dev main reporting environment:

Sent an event matching the shape of a Digital Letters event to my gaal8 eventbus' data plane:

Verified, using CloudWatch, that the event matched the rule and was successfully forwarded to the reporting repo:

Type of changes

• Refactoring (non-breaking change)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would change existing functionality)
• Bug fix (non-breaking change which fixes an issue)

Checklist

• I am familiar with the contributing guidelines
• I have followed the code style of the project
• I have added tests to cover my changes
• I have updated the documentation accordingly
• This PR is a result of pair or mob programming

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Sensitive Information Declaration

To ensure the utmost confidentiality and protect your and others privacy, we kindly ask you to NOT including PII (Personal Identifiable Information) / PID (Personal Identifiable Data) or any other sensitive data in this PR (Pull Request) and the codebase changes. We will remove any PR that do contain any sensitive information. We really appreciate your cooperation in this matter.

• I confirm that neither PII/PID nor sensitive data are included in this PR and the codebase changes.