Add Apache RAT for release-time license audit

[janhoy]

Summary

Apache RAT (Release Audit Tool) is required by ASF release policy to verify that every
file in a source release carries an approved license header. Add RAT and a .rat-excludes
(or rat-config.xml) exclusion list for files that are legitimately header-free (test
fixtures, generated files, binary assets, etc.).

Motivation

• ASF release policy mandates a clean RAT run before a release vote
• Apache Airflow maintains ~334 exclusion patterns as a reference
• JDK 21 is already a prerequisite for Solr, so the Maven/RAT toolchain is available

Proposed changes

1. Add a rat-config.xml (or .rat-excludes file) listing known exclusions
2. Add a Makefile target make rat that runs RAT against the source tree
3. Integrate the RAT check into the release checklist / CI release workflow
4. Document the step in DEVELOPER_GUIDE.md under a "Release" section

References

• https://creadur.apache.org/rat/
• ASF release policy: https://www.apache.org/legal/release-policy.html