[all] Please do not bump openstackdocstheme minimum version if possible

Hi,

The openstackdocstheme package in Debian is highly modified, because otherwise, building docs just fail. Namely, we have reverted commit d87aaca30f64502b3dd13cc1ddf46beec90fc015 because otherwise, the docs wouldn't build. Rebasing it is *very* annoying.

So, it'd be nice if packages didn't depend on the very latest version of openstackdocstheme so we could keep version 1.20.0. I don't think it's that important to depend on the very latest version (please let me know if I'm wrong, and let me know why).

Cheers,

Thomas Goirand (zigo)