dockerfile: Update base golang image to 1.20

Go 1.20 was recently released so this gets us back on a supported golang
version when building via the container.

Signed-off-by: John Mulligan <jmulligan@redhat.com>

Dockerfile

@@ -1,5 +1,5 @@
 # Build the manager binary
-FROM docker.io/golang:1.19 as builder
+FROM docker.io/golang:1.20 as builder
 ARG GIT_VERSION="(unset)"
 ARG COMMIT_ID="(unset)"
 ARG ARCH=""