[SAC] Gogs url moved to https://git.osgeo.org/gogs

[Sandro Santilli]

While the https://git.osgeo.org/_gogs_ url will keep working
I made the https://git.osgeo.org/gogs available and "primary"
url for the Gogs service.

Given GitLab deploy isn't happening, I though a more comfortable
url would have been nice for current users.

Some stats: 28 OSGeo users have logged in, there are 9 organizations
and 28 repositories registered.

Ref: https://wiki.osgeo.org/wiki/SAC:Gogs

--strk;