bump minimal Go version to 1.13

All our downstream platforms now support Go 1.13:

RHEL 8.2: golang-1.13.4
Fedora 31: golang-1.13.14

There's no reason anymore to stay on 1.12, therefore this commit bumps
the minimal required Go version to 1.13

.github/workflows/tests.yml

@@ -16,10 +16,10 @@ jobs:
     runs-on: ubuntu-latest
     steps:
 
-      - name: Set up Go 1.12
+      - name: Set up Go 1.13
         uses: actions/setup-go@v1
         with:
-          go-version: 1.12
+          go-version: 1.13
         id: go
 
       - name: Check out code into the Go module directory
@@ -69,10 +69,10 @@ jobs:
         ports:
           - 8080:80
     steps:
-      - name: Set up Go 1.12
+      - name: Set up Go 1.13
         uses: actions/setup-go@v1
         with:
-          go-version: 1.12
+          go-version: 1.13
         id: go
 
         # We need python for koji client.

README.md

@@ -50,7 +50,7 @@ The requirements for this project are:
 
 At build-time, the following software is required:
 
- * `go >= 1.12`
+ * `go >= 1.13`
  * `python-docutils >= 0.13`
 
 ### Build

go.mod

@@ -1,6 +1,6 @@
 module github.com/osbuild/osbuild-composer
 
-go 1.12
+go 1.13
 
 require (
 	github.com/Azure/azure-sdk-for-go v41.3.0+incompatible

tools/prepare-source.sh

@@ -2,7 +2,7 @@
 
 set -eux
 
-GO_VERSION=1.12.17
+GO_VERSION=1.13.15
 GO_BINARY=$(go env GOPATH)/bin/go$GO_VERSION
 
 # this is the official way to get a different version of golang