ci: build iceberg processor against v1.6.0

[novatechflow]

Summary

• bump the iceberg processor platform module requirement from v1.5.0 to v1.6.0
• update the release-build Dockerfile preload to download github.com/KafScale/platform@v1.6.0

Evidence

• release-image run 26826141334 failed in the iceberg processor image build
• the job log reported: no required module provides package github.com/KafScale/platform/pkg/lfs
• the addon still required github.com/KafScale/platform v1.5.0 while importing pkg/lfs, which was added after v1.5.0

Testing

• verified the addon go.mod and Dockerfile now reference v1.6.0 consistently for the release-image path