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.