Revert meld update to 3.21.0. This version requires pygobject 3.30
which is not present in the ports tree and fails to start at runtime.

Bump PORTEPOCH.

Reviewed by: bapt
Approved by: bapt, portmgr (fixit)
MFH: 2020Q4
Differential Revision: https://reviews.freebsd.org/D26775