Add Texas backup tag to dynamoDB (#1021)

# Task Branch Pull Request

**<https://nhsd-jira.digital.nhs.uk/browse/DS-2362>**

## Description of Changes

Adds a tag on the DynamoDB table to indicate we won't be using Texas's
backup solution. We covered by PiTR (Point in Time Recovery)

## Type of change

- New feature (non-breaking change which adds functionality)


## Development Checklist

- [x] I have performed a self-review of my own code
- [x] Tests have added that prove my fix is effective or that my feature
works (Integration tests)
- [x] I have updated Dependabot to include my changes (if applicable)

## Code Reviewer Checklist

- [x] I can confirm the changes have been tested or approved by a tester

Author: mabe13

infrastructure/stacks/shared-resources/dynamodb.tf

@@ -40,6 +40,10 @@ resource "aws_dynamodb_table" "message-history-table" {
     enabled = true
   }
 
+  tags = {
+    "backup_plan" = "none"
+  }
+
   depends_on = [
     aws_kms_key.signing_key
   ]