Adding a default for map.properties 'color.Impassable'

[LaFayette]

The following PR is up: https://github.com/triplea-game/triplea/pull/7637/

It will add the following default to map.properties

color.Impassable=DEB887

The majority of maps use that value. After the PR is merged the 'color.Impassable' property will become optional in map.properties, when not specified it will take the above default value.