Remove AL2023 migration helper

[0lut]

Summary

• remove the one-off AL2023 downtime migration helper now that PR Migrate ECS host default to AL2023 #48 has been merged and applied
• trim the deployment docs to steady-state pinned AL2023 AMI guidance
• point migration/validation history to PR Migrate ECS host default to AL2023 #48 instead of keeping rollback/migration runbook artifacts in-tree

Validation

• bash -n scripts/aws/deploy-ecs-ec2-ebs.sh scripts/aws/deploy-preview.sh scripts/aws/deploy-and-smoke.sh scripts/aws/smoke-test.sh scripts/aws/destroy-preview.sh scripts/aws/bootstrap.sh
• git diff --check

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.