Let's look at the available packages in the Ubuntu repositories for libstdc++6
:
$ rmadison libstdc++6 // The rmadison command is provided by the 'devscripts' package
libstdc++6 | 4.6.3-1ubuntu5 | precise | amd64, armel, armhf, i386, powerpc
libstdc++6 | 4.8.2-19ubuntu1 | trusty | amd64, arm64, armhf, i386, powerpc, ppc64el
libstdc++6 | 4.8.4-2ubuntu1~14.04.4 | trusty-security | amd64, arm64, armhf, i386, powerpc, ppc64el
libstdc++6 | 4.8.4-2ubuntu1~14.04.4 | trusty-updates | amd64, arm64, armhf, i386, powerpc, ppc64el
libstdc++6 | 5.3.1-14ubuntu2 | xenial | amd64, arm64, armhf, i386, powerpc, ppc64el, s390x
libstdc++6 | 5.4.0-6ubuntu1~16.04.10 | xenial-security | amd64, arm64, armhf, i386, powerpc, ppc64el, s390x
libstdc++6 | 5.4.0-6ubuntu1~16.04.11 | xenial-proposed | amd64, arm64, armhf, i386, powerpc, ppc64el, s390x
libstdc++6 | 5.4.0-6ubuntu1~16.04.11 | xenial-updates | amd64, arm64, armhf, i386, powerpc, ppc64el, s390x
libstdc++6 | 8-20180414-1ubuntu2 | bionic | amd64, arm64, armhf, i386, ppc64el, s390x
libstdc++6 | 8.2.0-1ubuntu2~18.04 | bionic-updates | amd64, arm64, armhf, i386, ppc64el, s390x
libstdc++6 | 8.2.0-7ubuntu1 | cosmic | amd64, arm64, armhf, i386, ppc64el, s390x
libstdc++6 | 8.2.0-12ubuntu1 | disco | amd64, arm64, armhf, i386, ppc64el, s390x
libstdc++6 | 8.2.0-13ubuntu1 | disco-proposed | amd64, arm64, armhf, i386, ppc64el, s390x
Aha. Neither of your two options, 8.1.0-5ubuntu1~14.04 nor 8.2.0-9 is from the Ubuntu repositories.
Now we know the problem: You have unwisely added non-Ubuntu sources that are providing packages that are incompatible with your release of Ubuntu. This is also called a 'version conflict' or 'held broken packages'. They are not really "broken", just wrong version...but they do break your system.
The solution is to uninstall ALL packages from that unwise source, and then to delete that source. Return your system to using packages provided by the Ubuntu repositories. Then MySQL will install.
If you don't know how to uninstall packages from a specific source, then see How can PPAs be removed? for instructions on how to install and use the ppa-purge
command.