Drop FreeBSD 11
Support for FreeBSD 11.4, the last FreeBSD 11.x release, ended on September 30, 2021 and the image can't be build without a workaround anymore.
The --with-readline
./configure
option has been added to gcc:sid:amd64
CI job; otherwise, it would be lost with the FreeBSD 11 removal.
This complements isc-projects/images!148.