Applies perfsprint linter (#24999)

Replacing usages of fmt.Sprint with faster alternatives

Signed-off-by: Philippe Antoine <contact@catenacyber.fr>

server/platform/services/searchengine/bleveengine/testlib.go

@@ -4,7 +4,7 @@
 package bleveengine
 
 import (
-	"fmt"
+	"strconv"
 
 	"github.com/mattermost/mattermost/server/public/model"
 )
@@ -13,7 +13,7 @@ func createPost(userId string, channelId string) *model.Post {
 	post := &model.Post{
 		Message:       model.NewRandomString(15),
 		ChannelId:     channelId,
-		PendingPostId: model.NewId() + ":" + fmt.Sprint(model.GetMillis()),
+		PendingPostId: model.NewId() + ":" + strconv.FormatInt(model.GetMillis(), 10),
 		UserId:        userId,
 		CreateAt:      1000000,
 	}