mail/stalwart: update to 0.16.18 release (+)
Release notes: https://github.com/stalwartlabs/stalwart/releases/tag/v0.16.16 \
https://github.com/stalwartlabs/stalwart/releases/tag/v0.16.17 \ https://github.com/stalwartlabs/stalwart/releases/tag/v0.16.18
Approved by: maintainer
PR: 297435