security/botan2: Update to 2.11.0

Changes: https://botan.randombit.net/news.html#version-2-11-0-2019-07-01

PR: 240557
Submitted by: Ralf van der Enden <tremere@cainites.net> (maintainer)
Approved by: araujo (mentor)
Differential Revision: https://reviews.freebsd.org/D21716