Update to go 1.20

[kousu]

Gitea is now testing against this, so I think we should too, as in #6.

[kousu]

As a reminder, on Ubuntu (+ most mainstream linuxes?) go 1.20 is too new, so you have to do

go install golang.org/dl/go1.20.3@latest
go1.20.3 download
ln -s go1.20.3 ~/go/bin/go

to get it. And if so then make sure you have done this too:

cat >> ~/.profile <<EOF
# set PATH so it includes go
if [ -d "$HOME/go/bin" ] ; then
    PATH="$HOME/go/bin:$PATH"
fi
EOF

[mguaypaq]

Thanks for the installation instructions!