chore(deps): update golang docker tag to v1.21 (#89)

This PR contains the following updates:

| Package | Type | Update | Change |
|---|---|---|---|
| golang |  | minor | `1.20` -> `1.21` |
| golang | stage | minor | `1.20` -> `1.21` |

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Reviewed-on: https://codeberg.org/woodpecker-plugins/docker-buildx/pulls/89

.woodpecker.yml

@@ -1,5 +1,5 @@
 variables:
-    - &golang 'golang:1.20'
+    - &golang 'golang:1.21'
     - &build_plugin 'woodpeckerci/plugin-docker-buildx:2'
     - base_settings: &base_buildx_settings
         platforms: 'linux/amd64,linux/arm64'

Dockerfile.multiarch

@@ -1,6 +1,6 @@
 ARG BUILDX_VERSION=0.11.0
 ARG DOCKER_VERSION=20.10-dind
-ARG GOLANG_VERSION=1.20
+ARG GOLANG_VERSION=1.21@sha256:24a09375a6216764a3eda6a25490a88ac178b5fcb9511d59d0da5ebf9e496474
 
 FROM --platform=$BUILDPLATFORM golang:${GOLANG_VERSION} as build