Update Elastic-Agent to 8.15.4

Summary

Requesting application be updated to a newer version.

Version Information

Current version: 8.15.3

Updated version: 8.15.4

Under support: the updated version is within the same major version of the application

Definition of Done

Hardening:

• Container builds successfully
• Container version has been updated in greylist file
• Branch has been merged into development

Justifications:

• All findings have been justified per the above documentation
• Justifications have been provided to the container hardening team

Approval Process:

• Peer review from Container Hardening Team
• Findings Approver has reviewed and approved all justifications
• Approval request has been sent to Authorizing Official
• Approval request has been processed by Authorizing Official

/cc @ironbank-notifications/updates