Fixes reference to elasticache instead of opensearch in AWS opensearch ingress rule

What does this MR do?

Fixed a typo in the ingress rule for AWS Opensearch referencing the Elasticache security group instead of the AWS Opensearch one. Presented as the following error:

╷
│ Error: Invalid index
│ 
│   on ../../modules/gitlab_ref_arch_aws/security.tf line 468, in resource "aws_vpc_security_group_ingress_rule" "gitlab_opensearch_internal_networking":
│  468:   security_group_id = aws_security_group.gitlab_elasticache_redis_cache[0].id
│     ├────────────────
│     │ aws_security_group.gitlab_elasticache_redis_cache is empty tuple
│ 
│ The given key does not identify an element in this collection value: the collection has no elements.
╵

Related issues

#569 (closed)

Author's checklist

When ready for review, the Author applies the workflowready for review label and mention @gl-quality/get-maintainers:

• Merge request:
  • Corresponding Issue raised and reviewed by the GET maintainers team.
  • Merge Request Title and Description are up-to-date, accurate, and descriptive
  • MR targeting the appropriate branch
  • MR has a green pipeline
  • MR has no new security alerts in the widget from the Secret Detection and IaC Scan (SAST) jobs.
• Code:
  • Check the area changed works as expected. Consider testing it in different environment sizes (1k,3k,10k,etc.).
  • Documentation created/updated in the same MR.
  • If this MR adds an optional configuration - check that all permutations continue to work.
  • For Terraform changes: set up a previous version environment, then run a terraform plan with your new changes and ensure nothing will be destroyed. If anything will be destroyed and this can't be avoided please add a comment to the current MR.
• Create any follow-up issue(s) to support the new feature across other supported cloud providers or advanced configurations. Create 1 issue for each provider/configuration. Contact the Quality Enablement team if unsure.