Migrate Dockerfile to use golang 1.21 to match go.mod in project

Author: huxcrux

Dockerfile

@@ -13,7 +13,7 @@
 # limitations under the License.
 
 # Build the manager binary
-FROM golang:1.20.10 as builder
+FROM golang:1.21.6 as builder
 WORKDIR /workspace
 
 # Run this with docker build --build_arg goproxy=$(go env GOPROXY) to override the goproxy