security/lego: Update to 4.3.1

Changes: https://github.com/go-acme/lego/blob/master/CHANGELOG.md#v431---2021-03-12
PR: 254228
Submitted by: Matthew Horan <matt@matthoran.com> (maintainer)
Approved by: fernape (mentor)
Differential Revision: https://reviews.freebsd.org/D29241