Install gcc/g++ 6 on Ubuntu 21.04

I am trying to compile Cuda 10.2 and predictably gcc-10 will not work to compile it. When using 20.04 it was simple, add bionic repo, update, install, update-alternatives. Using 21.04 I cannot get anything to work.

I tried with both xenial and bionic 'main' and 'universe' repos, which throws an error about the PUBKEY. I got the PUBKEYs from the ubuntu keyserver and update the package list, no errors.

sudo apt install g++-6

Package g++-6 is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

E: Package 'g++-6' has no installation candidate

sources.list ->

deb http://archive.ubuntu.com/ubuntu hirsute restricted main multiverse universe

deb http://archive.ubuntu.com/ubuntu hirsute-updates restricted main multiverse universe

deb http://archive.ubuntu.com/ubuntu hirsute-security restricted main multiverse universe

# gcc-6
deb [allow-insecure=yes] http://dk.archive.ubuntu.com/ubuntu/ bionic main universe
#deb [allow-insecure=yes] http://dk.archive.ubuntu.com/ubuntu/ xenial main universe

Does anyone know how I can achieve this using Ubuntu 21.04? Every post I have read says to add the bionic or xenial repos, update, install. I have had no luck so far.

I grabbed the packages (.deb) I needed from browsing the Ubuntu repos here.

Here is a list of the .deb packages I needed to manually install gcc/g++ 6. Install in the order of the list. This assumes you have most of the tools needed to compile and are just looking to switch default gcc versions.

sudo apt install ./libisl19_0.19-1_amd64.deb
sudo apt install ./gcc-6-base_6.4.0-17ubuntu1_amd64.deb
sudo apt install ./cpp-6_6.4.0-17ubuntu1_amd64.deb

# you can grab this next package from 21.04 repos, it handles a lot of the dependancies.
sudo apt install libgcc-6-dev

# Finally
sudo apt install ./gcc-6_6.4.0-17ubuntu1_amd64.deb

# GCC-6 is now installed, you can test by gcc-6 -v
baudneo@ZMES-test:~$ gcc-6 -v
gcc version 6.4.0 20180424 (Ubuntu 6.4.0-17ubuntu1)

# Now for G++ 6
sudo apt install ./libstdc++-6-dev_6.4.0-17ubuntu1_amd64.deb
sudo apt install ./g++-6_6.4.0-17ubuntu1_amd64.deb

# G++-6 is now installed! test by g++-6 -v
baudneo@ZMES-test:~$ g++-6 -v
gcc version 6.4.0 20180424 (Ubuntu 6.4.0-17ubuntu1)

# Now it is time to configure the system to use GCC G++ 6
# This assumes you do not have other versions of gcc and g++ installed for other projects

sudo update-alternatives --remove-all gcc 
sudo update-alternatives --remove-all g++
sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-6 100
sudo update-alternatives --set cc /usr/bin/gcc
sudo update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-6 100
sudo update-alternatives --set c++ /usr/bin/g++
# When you want to revert these back to default gcc-10
sudo update-alternatives --remove-all gcc 
sudo update-alternatives --remove-all g++
sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-10 100
sudo update-alternatives --set cc /usr/bin/gcc
sudo update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-10 100
sudo update-alternatives --set c++ /usr/bin/g++