[v2.12.0] Ensure CI/documentation reflect changes to default admin credentials

[derek-ho]

Background

Previously, when installing the security plugin demo configuration, the cluster was spun up with the default admin credentials, admin:admin. A change was made in main and backported to 2.x for the 2.12.0 release, which now requires an initial admin password to be passed in via the environment variable OPENSEARCH_INITIAL_ADMIN_PASSWORD. This will break some CI/testing that relies on OpenSearch to come up without setting this environment variable. This tracking issue is to ensure compliance with the new changes.

Coming from: opensearch-project/security#3624

Acceptance Criteria

• All documentation references to the old default credentials admin:admin are removed
• Ensure that CI/testing is working with main and 2.x branches

[prudhvigodithi]

[Triage]
Adding @gaiksaya @peterzhuamazon, we should update the dockerhub and ECR repo notes.

[derek-ho]

@prudhvigodithi I checked both dockerhub and ECR and seems like it was updated - thanks for taking that on! Closing this issue as completed, feel free to re-open if anything outstanding