libstdc++: Update expiry times for leap seconds lists

The list in tzdb.cc isn't the only hardcoded list of leap seconds in the
library, there's the one defined inline in <chrono> (to avoid loading
the tzdb for the common case) and another in a testcase. This updates
them to note that there are no new leap seconds in 2024 either, until at
least 2024-12-28.

libstdc++-v3/ChangeLog:

	* include/std/chrono (__get_leap_second_info): Update expiry
	time for hardcoded list of leap seconds.
	* testsuite/std/time/tzdb/leap_seconds.cc: Update comment.