Update golang Docker tag to v1.21

This MR contains the following updates:

Package	Type	Update	Change
golang	docker	minor	1.20-alpine -> 1.21-alpine

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.drone.yml

@@ -13,7 +13,7 @@ workspace:
 steps:
 - name: build
   pull: default
-  image: golang:1.20-alpine
+  image: golang:1.21-alpine
   environment:
     GOPROXY: https://proxy.golang.org
   commands: