Bump golang from 1.20-alpine to 1.21-alpine

Bumps golang from 1.20-alpine to 1.21-alpine.

---
updated-dependencies:
- dependency-name: golang
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

Dockerfile

@@ -10,7 +10,7 @@ RUN yarn build && yarn cache clean
 
 # Statping Golang BACKEND building from source
 # Creates "/go/bin/statping" and "/usr/local/bin/sass" for copying
-FROM golang:1.20-alpine AS backend
+FROM golang:1.21-alpine AS backend
 LABEL maintainer="Statping-NG (https://github.com/statping-ng)"
 ARG VERSION
 ARG COMMIT