build: make Go 1.18 the min version, update to Go 1.19

As usual, we keep with the last two Go versions, which means the last
year of Go release (they release every 6 months or so).

This change means we can now use the type params (generics) in the
project!

lnrpc/Dockerfile

@@ -1,4 +1,4 @@
-FROM golang:1.17.3-buster
+FROM golang:1.19-buster
 
 RUN apt-get update && apt-get install -y \
   git \