Update KCP library
There's been some new development in the library this year that seems potentially worth integrating: https://github.com/xtaci/kcp-go/compare/v5.6.3...v5.6.11
There's been some new development in the library this year that seems potentially worth integrating: https://github.com/xtaci/kcp-go/compare/v5.6.3...v5.6.11