build: bump golang from 1.20.12 to 1.21.5

Bumps golang from 1.20.12 to 1.21.5.

---
updated-dependencies:
- dependency-name: golang
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]

.github/workflows/assets.yml

@@ -17,7 +17,7 @@ jobs:
 
       - uses: actions/setup-go@v5
         with:
-          go-version: "1.20"
+          go-version: "1.21"
 
       - name: Build release assets
         run: make release

.github/workflows/tests.yml

@@ -23,7 +23,7 @@ jobs:
 
       - uses: actions/setup-go@v5
         with:
-          go-version: "1.20"
+          go-version: "1.21"
 
       - name: Install dependencies
         run: make get-deps

Dockerfile.alpine

@@ -1,7 +1,7 @@
 ARG DOCKER_GEN_VERSION=main
 
 # Build docker-gen from scratch
-FROM golang:1.20.12-alpine as go-builder
+FROM golang:1.21.5-alpine as go-builder
 
 ARG DOCKER_GEN_VERSION
 WORKDIR /build

Dockerfile.debian

@@ -1,7 +1,7 @@
 ARG DOCKER_GEN_VERSION=main
 
 # Build docker-gen from scratch
-FROM golang:1.20.12 as go-builder
+FROM golang:1.21.5 as go-builder
 
 ARG DOCKER_GEN_VERSION
 

go.mod

@@ -1,6 +1,6 @@
 module github.com/nginx-proxy/docker-gen
 
-go 1.20
+go 1.21
 
 require (
 	github.com/BurntSushi/toml v1.3.2