[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: #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

[stephen-crawford]

[Triage] Hi @derek-ho, could you check whether this is completed? Thank you! Otherwise looks good.

[derek-ho]

Integ test failures causing autocuts. Hopefully resolved with this PR: opensearch-project/opensearch-build#4331, but will keep this issue updated.

[derek-ho]

Original two acceptance criteria fulfilled, but i am waiting to close this until we resolve the opensearch-build repo integtest workflow to unblock all plugins.