smoktest: Add Rocky-8-EC2-Base back to OS test

Rocky-8-EC2-Base has been deprecated and no longer appears in AMI marketplace.

```
-> Adding tfvar aws_os="Rocky-8-EC2-Base"
-> Adding tfvar stack_version="9.3.6-SNAPSHOT"
-> Applying terraform configuration...

Error: Your query returned no results. Please change your search criteria and try again.

with module.standalone_apm_server.data.aws_ami.os,
on ../../infra/terraform/modules/standalone_apm_server/main.tf line 104, in data "aws_ami" "os":
104: data "aws_ami" "os" {
```

AC:
- Either remove it or find some replacement.
- Re-enable https://github.com/elastic/apm-server/actions/workflows/smoke-tests-os-sched.yml after fixing.

Workflow: https://github.com/elastic/apm-server/actions/runs/27323831894/job/80720571810