[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

[jackiehanyang]

There're two other issues with AD 2.x branch build:

• gradle version not compatible with Java20 - https://github.com/opensearch-project/anomaly-detection/actions/runs/7186078710/job/19570706034
• non compatible jackson-core version - https://github.com/opensearch-project/anomaly-detection/actions/runs/7350161068/job/20550526202

The first issue can be resolved after this pr is merged - #1132. Added a comment in this pr to include the jackson-core change here - #1132 (comment)

[jackiehanyang]

opened pr to remove default credentials - #1134