Switch to gexiv2_metadata_update_gps_info instead of set_gps_info

[felixc]

Bug report https://gitlab.gnome.org/GNOME/gexiv2/-/issues/53 points out that the older gexiv2_metadata_set_gps_info overwrites other GPS metadata fields as an undesired side effect. gexiv2_metadata_update_gps_info was added in gexiv2 version 0.12.1 from May 2020, and behaves more like one would expect -- only overwriting the fields that are being changed.

We should switch to using that, but first gexiv2-sys will need to add support for it, and likely a check for what version of gexiv2 is installed.