Maintenance Upgrades
- Bump to OpenSearch 3.9.0 by @rursprung in #614
- Move files around and add SPDX license headers by @msfroh in #595
- Update compatibility matrix: add 2.19.4/5/6 rows and fix 3.8.0.0 date by @ps48 in #600
Full Changelog: 3.8.0.0...3.9.0.0