dockerfile: bump builder to 'golang:1.21'

Required minimal version or else 'make check-dockerfile-go-version'
fails.

Signed-off-by: Shachar Sharon <[email protected]>

Author: synarete

Dockerfile

@@ -1,5 +1,5 @@
 # Build the manager binary
-FROM docker.io/golang:1.20 as builder
+FROM docker.io/golang:1.21 as builder
 ARG GIT_VERSION="(unset)"
 ARG COMMIT_ID="(unset)"
 ARG ARCH=""