[MM-16562] Migrate User.GetUnreadCountForChannel to Sync by default (#11604)

* Migrate User.GetUnreadCountForChannel to Sync by default

* Remove else block from GetUnreadCountChannel in user_store.go

* Return nil value for count when an error occurs in GetUnreadCountForChannel

* Fix linting issue in user_store.go

store/storetest/mocks/CommandWebhookStore.go

@@ -4,8 +4,8 @@
 
 package mocks
 
-import "github.com/stretchr/testify/mock"
-import "github.com/mattermost/mattermost-server/model"
+import mock "github.com/stretchr/testify/mock"
+import model "github.com/mattermost/mattermost-server/model"
 
 // CommandWebhookStore is an autogenerated mock type for the CommandWebhookStore type
 type CommandWebhookStore struct {