gs-utils: Let gs_utils_get_file_age() return guint64

Since !1219 (merged) the cache file age is turned into guint64. Having the gs_utils_get_file_age() return guint and the cache file not existing meant returning G_MAXUINT, which is smaller than G_MAXUINT64, thus when the file did not exist it was behaving like the file exists and it failed later in the code.

This could be seen on start as a runtime warning when there's no ratings.json file in the cache and automatic updates are enabled.