Backport PR #17063 to 8.17: Use centralized source of truth for active branches

[dliappis]

Backport PR #17063 to 8.17 branch, original message:

---

Release notes

[rn:skip]

What does this PR do?

This commit simplifies the DRA process in Logstash by removing the need to maintain a separate file for the active branches, and instead rely on a centrally maintained file containing source of truth.
While at it, we refactor/simplify the creation of an array with the versions in .buildkite/scripts/snyk/resolve_stack_version.sh.

Why is it important/What is the impact to the user?

Removes a manual step required when new branches get cut.

How to test this PR locally

$ PIPELINES_TO_TRIGGER="test-pipeline" .buildkite/scripts/common/trigger-pipeline-generate-steps.sh 
--- Printing generated steps
steps:
  - trigger: test-pipeline
    label: ":testexecute: Triggering test-pipeline / 7.17"
    build:
      branch: "7.17"
      message: ":testexecute: Scheduled build for 7.17"
  - trigger: test-pipeline
    label: ":testexecute: Triggering test-pipeline / 8.x"
    build:
      branch: "8.x"
      message: ":testexecute: Scheduled build for 8.x"
  - trigger: test-pipeline
    label: ":testexecute: Triggering test-pipeline / 8.16"
    build:
      branch: "8.16"
      message: ":testexecute: Scheduled build for 8.16"
  - trigger: test-pipeline
    label: ":testexecute: Triggering test-pipeline / 8.17"
    build:
      branch: "8.17"
      message: ":testexecute: Scheduled build for 8.17"
  - trigger: test-pipeline
    label: ":testexecute: Triggering test-pipeline / 8.18"
    build:
      branch: "8.18"
      message: ":testexecute: Scheduled build for 8.18"
  - trigger: test-pipeline
    label: ":testexecute: Triggering test-pipeline / 9.0"
    build:
      branch: "9.0"
      message: ":testexecute: Scheduled build for 9.0"
  - trigger: test-pipeline
    label: ":testexecute: Triggering test-pipeline / main"
    build:
      branch: "main"
      message: ":testexecute: Scheduled build for main"

$ .buildkite/scripts/snyk/resolve_stack_version.sh 
Fetching versions from https://storage.googleapis.com/artifacts-api/snapshots/branches.json
7.17 8.x 8.16 8.17 8.18 9.0 main

Test in buildkite by triggering the scheduled pipeline: https://buildkite.com/elastic/logstash-pipeline-scheduler/builds/568#0194fa84-0a66-42d7-8806-d4610ce2abce/64-100

[dliappis]

@mashhur same request as in #17073 (comment); compared to main and 9.0, in this branch the resolve_stack_version.sh script doesn't consult ci/branches.json so I left it as is.

[elastic-sonarqube]

Quality Gate passed

Issues
0 New issues
0 Fixed issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
No data about Duplication

See analysis details on SonarQube

[elasticmachine]

💚 Build Succeeded

• Buildkite Build
• Commit: cc4c47d

cc @dliappis

[mashhurs]

🎉 LGTM!