libstdc++: Fix value of __cpp_lib_format macro [PR111826]

As noted in the PR, we support both features required for the 202110L
value, so we should define it with that value.

libstdc++-v3/ChangeLog:

	PR libstdc++/111826
	* include/std/format (__cpp_lib_format): Update value.
	* include/std/version (__cpp_lib_format): Likewise.
	* testsuite/std/format/functions/format.cc: Update expected
	value.

(cherry picked from commit cdf45e00)