libstdc++: Update value of __cpp_lib_jthread macro

libstdc++: Update value of __cpp_lib_jthread macro

	* include/std/condition_variable (__cpp_lib_jthread): Remove
	redundant definition.
	* include/std/stop_token (__cpp_lib_jthread): Update macro value to
	indicate P1869R1 support.
	* include/std/version (__cpp_lib_jthread): Update value.
	* testsuite/30_threads/condition_variable_any/stop_token/1.cc: Check
	for updated macro value.
	* testsuite/30_threads/condition_variable_any/stop_token/2.cc:
	Likewise.
	* testsuite/30_threads/jthread/1.cc: Likewise.
	* testsuite/30_threads/jthread/2.cc: Likewise.
	* testsuite/30_threads/stop_token/1.cc: Likewise.
	* testsuite/30_threads/stop_token/2.cc: Likewise.