MM-49485: Set BoardsProduct FF to on by default (#22006)

https://mattermost.atlassian.net/browse/MM-49485

```release-note
Boards will be served as an in-built product from
within Mattermost server instead of a plugin.

If you want to disable this, please set the
MM_FEATUREFLAGS_BoardsProduct env var to "false" (without
                                                  the quotes).

In this mode, the boards plugin will remain disabled.
```

model/feature_flags.go

@@ -95,7 +95,7 @@ func (f *FeatureFlags) SetDefaults() {
 	f.InsightsEnabled = true
 	f.CommandPalette = false
 	f.CallsEnabled = true
-	f.BoardsProduct = false
+	f.BoardsProduct = true
 	f.SendWelcomePost = true
 	f.PostPriority = true
 	f.PeopleProduct = false