[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: https://github.com/opensearch-project/security/issues/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

[r1walz]

Closing as already addressed.