mail/aerc: update to 0.21.0
Changelog: https://git.sr.ht/~rjarry/aerc/refs/0.21.0
PR: 289162
Reported by: Herbert J. Skuhra <herbert@gojira.at>
Reviewed by: Yuvakiran Rayapureddy <iamsleepy@ryuki.me>
Approved by: Yuvakiran Rayapureddy <iamsleepy@ryuki.me> (maintainer)