lnrpc: avoid needing to download Golang

Because the Go version used to run the `go list` commands is below the
minimum version specified in the main go.mod file, every time the `make
rpc` command is executed, the Golang runtime is downloaded twice, which
looks like this and takes a couple of seconds at least:

go: downloading go1.21.4 (linux/amd64)
go: downloading go1.21.4 (linux/amd64)

We fix this by using the correct minimum version.

lnrpc/gen_protos_docker.sh

@@ -6,7 +6,7 @@ set -e
 DIR="$(cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd)"
 
 # golang docker image version used in this script.
-GO_IMAGE=docker.io/library/golang:1.21.0-alpine
+GO_IMAGE=docker.io/library/golang:1.21.4-alpine
 
 PROTOBUF_VERSION=$(docker run --rm -v $DIR/../:/lnd -w /lnd $GO_IMAGE \
   go list -f '{{.Version}}' -m google.golang.org/protobuf)