Reduce bridge tower materials from 400 to 300 hcms.

Reduce bridge tower materials from 400 to 300 hcms.

Bridge spans have been unable to produce enough avail for a tower with
default game configuration since we reduced their maximum population
in commit 6bbd7ffd, v4.3.6.  This commit reduces required avail from
160 to 120, fixing the stock game: 100 civilians and 100 uws can make
that much in a 60 ETU update.