ci: use go1.21.0 fmt to make ci happy

https://github.com/prometheus/prometheus/actions/runs/6044443719/job/16403043771?pr=12774

Signed-off-by: TJ Hoplock <t.hoplock@gmail.com>

notifier/notifier.go

@@ -31,8 +31,8 @@ import (
 	"github.com/prometheus/alertmanager/api/v2/models"
 	"github.com/prometheus/client_golang/prometheus"
 	config_util "github.com/prometheus/common/config"
-	"github.com/prometheus/common/sigv4"
 	"github.com/prometheus/common/model"
+	"github.com/prometheus/common/sigv4"
 	"github.com/prometheus/common/version"
 	"go.uber.org/atomic"