glib: Format GDateTime ISO8601 years as %C%y
The current use of %Y does not take into account that %Y will not pad the year with 0's, meanwhile ISO8601 does expect a full YYYY format for all dates. This breaks the formatting with dates prior to the year 1000.
Split this into %C%y so 2-digit, 0-padded century and year are printed separately, this gives the expected YYYY format.