CDPS-984: Downgrading back to redis t4g.micro after testing

[sablumiah]

Terraform Plan Summary

<details "open">

Terraform Plan: 0 to be created, 0 to be destroyed, 3 to be updated, 0 to be replaced and 9 unchanged.

Resources to update:

! module.elasticache_redis.aws_elasticache_replication_group.ec_redis
! module.elasticache_redis.aws_iam_policy.irsa
! pingdom_check.hmpps-micro-frontend-components-preprod-check

[cloud-platform-concourse-bot]

Your PR is applying in the build: https://concourse.cloud-platform.service.justice.gov.uk/teams/main/pipelines/environments-live/jobs/apply-namespace-changes-live/builds/8023