databases/couchdb2: update to 2.3.1

• explicitly depend on OTP>=21.3 to skip BEAM coredumps observed in the wild
• drop OTP<20 option per above
• tweak rc.d script to ensure coredumps can be retrieved if needed

Approved by: jrm (mentor)
Differential Revision: https://reviews.freebsd.org/D19621