"Open" should be the default PR query #2233

Found in https://github.com/microsoft/vssaas-planning/issues/1529 -> https://github.com/microsoft/vssaas-planning/issues/2176

Currently, there is no default query for "Open" PR's. Given that is the default in GitHub, it should be a default query as well. Otherwise "All" is the only view that lets you see PR's and non-power users won't know to adjust the query.