Increase our Sphinx minimum to 1.8

Since I11f165f59c099f3ab4a2fe4fcd0cb421cfd43285 has added
html_baseurl to our Sphinx config for RFC 5988 support, which is
only implemented in 1.8 and later, increase the minimum version we
declare as needed in the doc/requirements.txt file.

Change-Id: I264bb9e52fac31b093b3e8b343446a0ff3a02da4

doc/requirements.txt

@@ -2,7 +2,7 @@ ansible
 # NOTE(ianw) 2021-07-13: <4.1.0 until related issue in
 # sphixcontrib-httpdomain fixed
 #  https://github.com/sphinx-contrib/httpdomain/pull/51
-sphinx>=1.6.1,<4.1.0
+sphinx>=1.8,<4.1.0
 sphinxcontrib-blockdiag>=1.1.0
 sphinxcontrib-programoutput
 sphinx-autodoc-typehints