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.
```

config/diff_test.go

@@ -808,7 +808,7 @@ func TestDiff(t *testing.T) {
 							Enable: !defaultConfigGen().PluginSettings.PluginStates["com.mattermost.nps"].Enable,
 						},
 						"focalboard": {
-							Enable: true,
+							Enable: false,
 						},
 						"playbooks": {
 							Enable: true,
@@ -846,7 +846,7 @@ func TestDiff(t *testing.T) {
 							Enable: true,
 						},
 						"focalboard": {
-							Enable: true,
+							Enable: false,
 						},
 						"playbooks": {
 							Enable: true,
@@ -876,7 +876,7 @@ func TestDiff(t *testing.T) {
 					BaseVal: defaultConfigGen().PluginSettings.PluginStates,
 					ActualVal: map[string]*model.PluginState{
 						"focalboard": {
-							Enable: true,
+							Enable: false,
 						},
 						"playbooks": {
 							Enable: true,