devel/reflect-cpp: Update 0.20.0 => 0.21.0

Changelogs:

• https://github.com/getml/reflect-cpp/releases/tag/v0.21.0
• https://github.com/getml/reflect-cpp/compare/v0.20.0...v0.21.0

• Keep pkg-descr within 24 lines limit

Approved by: yuri@ (Mentor)

Differential Revision: https://reviews.freebsd.org/D52602