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

[ericywl]

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

[carsonip]

#21245 should be related and should be fixed together

[carsonip]

There is active discussion about missing 8.10 Rocky Linux issue on their community forum: https://forums.rockylinux.org/t/rocky-8-10-amis-missing-from-aws-eu-west-1-region/20558 . Marking as blocked

[carsonip]

Currently mitigated by #21275 . Similar situation to #20561