MM-52646: Fix data race on product notices (#23910)

The UpdateProductNotices method runs in a separate goroutine
which modifies a global structure called noticesCache.
This creates race condition when one test finishes
but the goroutine hasn't finished running.

To fix this, we simply prevent the spawning of the goroutine
by disabling the feature in the config.

Ideally, we'd not have global state in the first place.
But that's a separate matter.

https://mattermost.atlassian.net/browse/MM-52646

```release-note
NONE
```

server/channels/app/server.go

@@ -462,12 +462,12 @@ func NewServer(options ...Option) (*Server, error) {
 
 	// if enabled - perform initial product notices fetch
 	if *s.platform.Config().AnnouncementSettings.AdminNoticesEnabled || *s.platform.Config().AnnouncementSettings.UserNoticesEnabled {
-		go func() {
+		s.platform.Go(func() {
 			appInstance := New(ServerConnector(s.Channels()))
 			if err := appInstance.UpdateProductNotices(); err != nil {
 				mlog.Warn("Failed to perform initial product notices fetch", mlog.Err(err))
 			}
-		}()
+		})
 	}
 
 	if s.skipPostInit {

server/channels/app/server_test.go

@@ -42,6 +42,8 @@ func newServerWithConfig(t *testing.T, f func(cfg *model.Config)) (*Server, erro
 	store, err := config.NewStoreFromBacking(configStore, nil, false)
 	require.NoError(t, err)
 	cfg := store.Get()
+	*cfg.AnnouncementSettings.AdminNoticesEnabled = false
+	*cfg.AnnouncementSettings.UserNoticesEnabled = false
 	cfg.SqlSettings = *mainHelper.GetSQLSettings()
 	f(cfg)
 
@@ -110,6 +112,8 @@ func TestStartServerNoS3Bucket(t *testing.T) {
 		AmazonS3SSL:             model.NewBool(false),
 	}
 	*cfg.ServiceSettings.ListenAddress = "localhost:0"
+	*cfg.AnnouncementSettings.AdminNoticesEnabled = false
+	*cfg.AnnouncementSettings.UserNoticesEnabled = false
 	cfg.SqlSettings = *mainHelper.GetSQLSettings()
 	_, _, err := store.Set(cfg)
 	require.NoError(t, err)
@@ -191,6 +195,8 @@ func TestStartServerTLSVersion(t *testing.T) {
 	cfg := store.Get()
 	testDir, _ := fileutils.FindDir("tests")
 
+	*cfg.AnnouncementSettings.AdminNoticesEnabled = false
+	*cfg.AnnouncementSettings.UserNoticesEnabled = false
 	*cfg.ServiceSettings.ListenAddress = "localhost:0"
 	*cfg.ServiceSettings.ConnectionSecurity = "TLS"
 	*cfg.ServiceSettings.TLSMinVer = "1.2"