OGS version information is now handled by its git tag.

When the current commit is also a tag then the version is the tag name,
e.g. "6.0.1". When the current commit is not a tag then the ogs version
is the last tag + number of commits since then + current commit short
hash, e.g. "6.0.1-140-g80ac475".

This info is available in BaseLib::BuildInfo::git_describe. Use it to
uniquely set the OGS version.