Bump the openstackdocstheme extension to 1.20
Some options are now automatically configured by the version 1.20[1]:
- project
- html_last_updated_fmt
- latex_engine
- latex_elements
[1]https://docs.openstack.org/openstackdocstheme/latest/#using-the-theme
Change-Id: I514c232a0f0d0ca47031a13b325bbc98d2ca62db