Managed flag is set to false when exporting dashboards

[taylor-swanson]

When exporting dashboards, either brand new dashboard or an existing managed one made editable by elastic-package, the managed flag on the dashboard (within the exported JSON) is set to false. This may be desirable in some situations, but in most cases, these dashboards are meant for packages managed by fleet, so the desired value should be true.

At the moment, this doesn't seem to cause any issues, since it appears Fleet either overrides this value or ignores it completely. However, many (if not all) of our integrations have managed set to false in checked-in code. Another option would be to not include managed in the JSON at all, but not sure if that's the equivalent to "managed": false.

Another thing to note, adding the managed tag on a dashboard doesn't appear to set the managed flag in the JSON, so as of right now, the only way to set it to true is to edit the JSON.

[jsoriano]

I think we could remove managed when exporting dashboards, as Fleet will set this value.