Chore: [AEA-5803] - revert use but not creation of PSU secrets (#2442)

## Summary

- 🤖 Operational or Infrastructure Change

### Details

We need to create new PSU secrets one release ahead of using. Hence this
uses the old `secrets` stack.
Confluence page to go along with the release:
https://nhsd-confluence.digital.nhs.uk/spaces/APIMC/pages/1236043264/Migration+to+PSU+managed+secrets+for+NHS+Notify+integration

Author: tstephen-nhs

SAMtemplates/functions/main.yaml

@@ -431,12 +431,9 @@ Resources:
           LOG_LEVEL: !Ref LogLevel
           NHS_NOTIFY_PRESCRIPTIONS_SQS_QUEUE_URL: !Ref NHSNotifyPrescriptionsSQSQueueUrl
           TABLE_NAME: !Ref PrescriptionNotificationStatesTableName
-          API_KEY_SECRET:
-            Fn::ImportValue: !Sub ${StackName}-PSU-Notify-API-Key
-          PRIVATE_KEY_SECRET:
-            Fn::ImportValue: !Sub ${StackName}-PSU-Notify-PrivateKey
-          KID_SECRET:
-            Fn::ImportValue: !Sub ${StackName}-PSU-Notify-KID
+          API_KEY_SECRET: secrets-PSU-Notify-API-Key
+          PRIVATE_KEY_SECRET: secrets-PSU-Notify-PrivateKey
+          KID_SECRET: secrets-PSU-Notify-Application-KID
           NHS_NOTIFY_ROUTING_ID_PARAM: !Ref NotifyRoutingPlanIDParam
           NOTIFY_API_BASE_URL_PARAM: !Ref NotifyAPIBaseURLParam
           MAKE_REAL_NOTIFY_REQUESTS_PARAM: !Ref EnableNotificationsExternalParam
@@ -500,10 +497,8 @@ Resources:
         Variables:
           LOG_LEVEL: !Ref LogLevel
           TABLE_NAME: !Ref PrescriptionNotificationStatesTableName
-          APP_ID_SECRET:
-            Fn::ImportValue: !Sub ${StackName}-PSU-Notify-Application-ID
-          API_KEY_SECRET:
-            Fn::ImportValue: !Sub ${StackName}-PSU-Notify-API-Key
+          APP_ID_SECRET: secrets-PSU-Notify-Application-ID
+          API_KEY_SECRET: secrets-PSU-Notify-API-Key
     Metadata:
       BuildMethod: esbuild
       guard: