Switch letter_status_update lambda to use eventsub SNS topic

[stevebux]

Description

Context

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
• If I have used the 'skip-trivy-package' label I have done so responsibly and in the knowledge that this is being fixed as part of a separate ticket/PR.

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[Vlasis-Perdikidis]

for these clients we don't specify the region.
Are we sure they are assigned the correct region?

[stevebux]

As I understand it, when you create an AWS client inside a lambda, it inherits the region from where the lambda is executed. (Which will be eu-west-2 as that's what's configured in the terraform in the nhs-notify-internal project). I suppose that might be an issue for services that are only available in a limited number of regions, but that doesn't seem to be a problem here.