mrs: ftbfs with GCC-10

Note: This implicitly blocks boost1.71 which implicitly blocks ICU so a lot of things depend on it.
Hence Prio=High

The build error is present in Ubuntu
https://launchpad.net/ubuntu/+source/mrs/6.0.5+dfsg-7build8
https://launchpadlibrarian.net/490674525/buildlog_ubuntu-groovy-amd64.mrs_6.0.5+dfsg-7build8_BUILDING.txt.gz

and Debian:
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=957567

The package's upstream seems dead or at least non-updated
mrs | 6.0.5+dfsg-1 | oldoldstable | source, amd64
mrs | 6.0.5+dfsg-2 | oldstable | source
mrs | 6.0.5+dfsg-2+b2 | oldstable | amd64
mrs | 6.0.5+dfsg-7 | stable | source
mrs | 6.0.5+dfsg-7 | unstable | source
mrs | 6.0.5+dfsg-7 | unstable-debug | source
mrs | 6.0.5+dfsg-7+b2 | stable | amd64, arm64, armel, armhf, i386, mips, mips64el, mipsel, ppc64el, s390x
mrs | 6.0.5+dfsg-7+b4 | unstable | amd64, arm64, armel, armhf, i386, mips64el, mipsel, ppc64el, s390x
 mrs | 6.0.5+dfsg-1build2 | xenial/universe | source, amd64
 mrs | 6.0.5+dfsg-3ubuntu1 | bionic/universe | source, amd64, arm64, ppc64el, s390x
 mrs | 6.0.5+dfsg-7build2 | eoan/universe | source, amd64, arm64, armhf, i386, ppc64el, s390x
 mrs | 6.0.5+dfsg-7build7 | focal/universe | source, amd64, arm64, armhf, ppc64el, riscv64, s390x
 mrs | 6.0.5+dfsg-7build7 | groovy/universe | source, amd64, arm64, armhf, ppc64el, riscv64, s390x
 mrs | 6.0.5+dfsg-7build8 | groovy-proposed/universe | source

6.0.5 all the time.
Also please do note that due to the Debian bug this was already removed in Debian and if the FTBFS is complex that might be our way out as well.