Test with Go 1.21 and later

Something ends up requiring the slices package, which was added in Go
1.21.

Signed-off-by: Stephen Kitt <[email protected]>

.github/workflows/go.yml

@@ -9,7 +9,7 @@ jobs:
   build:
     strategy:
       matrix: 
-        go-versions: [1.16.x, 1.17.x, 1.18.x, 1.19.x]
+        go-versions: [1.21.x, 1.22.x, 1.23.x]
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v2