libstdc++: Update __cpp_lib_erase_if macro (P1115R3)

libstdc++: Update __cpp_lib_erase_if macro (P1115R3)

Now that this feature has been approved for C++20 we can define the
macro to the official value.

	* include/bits/erase_if.h (__cpp_lib_erase_if): Define to 202002L.
	* include/std/deque: Likewise.
	* include/std/forward_list: Likewise.
	* include/std/list: Likewise.
	* include/std/string: Likewise.
	* include/std/vector: Likewise.
	* include/std/version: Likewise.
	* testsuite/23_containers/deque/erasure.cc: Test for new value.
	* testsuite/23_containers/forward_list/erasure.cc: Likewise.
	* testsuite/23_containers/list/erasure.cc: Likewise.
	* testsuite/23_containers/map/erasure.cc: Likewise.
	* testsuite/23_containers/set/erasure.cc: Likewise.
	* testsuite/23_containers/unordered_map/erasure.cc: Likewise.
	* testsuite/23_containers/unordered_set/erasure.cc: Likewise.
	* testsuite/23_containers/vector/erasure.cc: Likewise.