cloud: migrate YDB UI deployments to a supported ALB controller

[astandrik]

Goal

Move the cloud deployments of YDB UI and its admin interface away from the legacy ALB Ingress Controller to a supported controller. Preserve availability and useful monitoring signals while enabling clearer error handling.

Plan

• Wait for the required upstream capabilities and confirm a suitable release before starting the pilot.
• Verify compatibility with existing behavior and identify migration prerequisites.
• Define the rollout, validation and rollback plan.
• Validate the migration in a pre-production pilot.
• Roll out gradually to the remaining in-scope deployments after the pilot is accepted.
• Update operational documentation and retire legacy resources after verification.

Acceptance

The migrated deployments retain their existing user-facing behavior. Error handling and monitoring are verified, and rollout results and rollback steps are documented. Do not combine this work with changes to alert thresholds or notification channels. KZ deployments are out of scope.

Related: #4177.

Reference: General Gwin migration guide.