[build] Include legal files in Fluss server image

[wuchong]

Purpose

Linked issue: close #4426

The Fluss server Docker image is built from build-target. A later mvn clean deploy in the release process regenerates that directory, but the binary assembly did not include LICENSE or NOTICE. As a result, the published server image omitted both files.

Brief change log

• Add LICENSE-bin and NOTICE-bin to the binary assembly as LICENSE and NOTICE.
• Remove the release script's post-package copy, so the assembly is the single source of truth.
• Compare the generated legal files byte-for-byte in CI and smoke-test their presence in the server image.
• Include the legal files in the workflow trigger and image cache key.
• Document the same content checks in the release guide.

Tests

• ./mvnw -B --no-transfer-progress clean package -pl fluss-dist -am -DskipTests
• ./mvnw -B --no-transfer-progress test -pl fluss-dist -am -Dtest=DistShellScriptTest -Dsurefire.failIfNoSpecifiedTests=false -DskipITs
• cmp LICENSE-bin build-target/LICENSE
• cmp NOTICE-bin build-target/NOTICE
• Verified the generated tar contains fluss-1.1-SNAPSHOT/LICENSE and fluss-1.1-SNAPSHOT/NOTICE.
• Built an offline verification image from the locally cached RC3 base and verified both container files and their SHA-256 hashes. The direct Dockerfile build could not refresh the base image because the local Docker registry mirror returned EOF; CI will exercise the normal build.
• bash -n tools/releasing/create_binary_release.sh
• Parsed .github/workflows/client-integration.yml as YAML.

API and Format

No API or storage format changes.

Documentation

The release guide now requires byte-for-byte comparison of the generated legal files before building the server image.