www/miniflux: needs Go 1.21

This port requires the Go package log/slog which is only available with
Go 1.21 or newer. Bump the Go toolchain version required to 1.21.

Approved by: portmgr (build fix blanket)
MFH: 2023Q4

(cherry picked from commit 7838bd81960f26d7c331864f732fe8950993515b)