go.mod: update to go v1.21

Go 1.21 is included in all currently supported distro versions.
It is required by new updates to some of our dependencies, including
osbuild/images.

.github/workflows/tests.yml

@@ -71,7 +71,7 @@ jobs:
     - uses: actions/checkout@v4
     - uses: actions/setup-go@v5
       with:
-        go-version: "1.20"
+        go-version: "1.21"
     - env:
         PGUSER: postgres
         PGPASSWORD: foobar
@@ -111,10 +111,10 @@ jobs:
     name: "⌨ Golang Lint"
     runs-on: ubuntu-latest
     steps:
-      - name: Set up Go 1.20
+      - name: Set up Go 1.21
         uses: actions/setup-go@v5
         with:
-          go-version: "1.20"
+          go-version: "1.21"
         id: go
 
       - name: Check out code into the Go module directory
@@ -158,10 +158,10 @@ jobs:
     runs-on: ubuntu-20.04
     steps:
 
-      - name: Set up Go 1.20
+      - name: Set up Go 1.21
         uses: actions/setup-go@v5
         with:
-          go-version: "1.20"
+          go-version: "1.21"
         id: go
 
       - name: Check out code into the Go module directory