I was looking for this same thing on last month and there isn´t much documentation about it, but the good part is that you can set it as the default landing page to all users directly on the workspace options. To do this open the workspace you want to share and on the menu select the "Share" menu item then select "Share Project" item and on the share settings box that opens at the bottom check the "Set as Landing Page for Recipients" checkbox. Its important to set as recipients the All users option (similar to the screenshot below).
Now the bad part here is the above solution won´t work with new users added, it will only work with users that are already on the tool, at least from what i have investigated on documentation there is no option yet for this to be configured by default for new users, so they will see the dashboards section the first time they login. As a recommendation what my team is doing for now is to everytime we give access to a new user we go to the workspace and rerun the setting again before letting our users know that they have access, this so they get workspaces instead of the legacy reports option.
I hope this helps and hopefully in the near future we will see an option for this.
