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
@@ -18,7 +18,7 @@ jobs:
       with:
         go-version: ${{ matrix.go-versions }}
     - name: Install
-      run: GO111MODULE=off go get golang.org/x/lint/golint
+      run: go install golang.org/x/lint/golint@latest
     - name: Run gofmt
       run: diff -u <(echo -n) <(gofmt -d *.go)
     - name: Run golint