docs/INSTALL.md: update min Go version to 1.18
docs/INSTALL.md: update min Go version to 1.18

In this commit, we update the min Go version to be version 1.18. This
breaks our prior rule of supporting the last two major versions, but
allows us to use the larger features in 1.18 sooner, such as the newly
released type parameters.

aakselrod/lnd,aakselrod/lnd