golang: Switch default to Go1.20

Since go1.18 is no longer supported, we should switch to a newer
version (("Each major Go release is supported until there are two
newer major releases" [1]).

[1] https://go.dev/doc/devel/release

/reviewed-by @kirr
/reviewed-on nexedi/slapos!1417

component/golang/buildout.cfg

@@ -177,7 +177,7 @@ bin	= ${gowork.dir:bin}
 depends = ${gowork.goinstall:recipe}
 
 # go version used for the workspace (possible to override in applications)
-golang  = ${golang1.18:location}
+golang  = ${golang1.20:location}
 
 # no special build flags by default
 buildflags =