tech: replace bmizerany/assert with stretchr/testify (#16625)

bmizerany is old and unsupported. so we are replacing it
with stretchr which is an drop in replacement and something
we want to use more in Grafana.

pkg/components/simplejson/simplejson_test.go

@@ -4,7 +4,7 @@ import (
 	"encoding/json"
 	"testing"
 
-	"github.com/bmizerany/assert"
+	"github.com/stretchr/testify/assert"
 )
 
 func TestSimplejson(t *testing.T) {