libstdc++: Support P2510R3 "Formatting pointers" as a DR for C++20

We already enable this for -std=gnu++20 but we can do it for -std=c++20
too. Both libc++ and MSVC also treat this change as a DR for C++20.

Now that the previous change to the value of __cpp_lib_format is
supported, we can finally update it to 202304 to indicate support for
this feature too.

libstdc++-v3/ChangeLog:

	* include/bits/version.def (format): Update value for P2510R3.
	* include/bits/version.h: Regenerate.
	* include/std/format (_GLIBCXX_P2518R3): Remove misspelled macro
	and check __glibcxx_format instead.
	* testsuite/std/format/functions/format.cc: Check value of the
	__cpp_lib_format macro for formatting pointers support.
	* testsuite/std/format/parse_ctx.cc: Likewise.