build: update Go to 1.21

.gitlab-ci.yml

-image: golang:1.20
+image: golang:1.21
 
 stages:
   - prepare

Dockerfile

 ### Build Image
-FROM golang:1.20-bookworm AS builder
+FROM golang:1.21-bookworm AS builder
 LABEL maintainer="Christian Pointner <equinox@helsinki.at>"
 
 RUN set -e \

go.mod

 module gitlab.servus.at/autoradio/tank
 
-go 1.20
+go 1.21
 
 require (
 	github.com/codegangsta/cli v1.22.14