Fix pg_truncate() on Windows.

Commit 57faaf376 added pg_truncate(const char *path, off_t length), but
"length" was ignored under WIN32 and the file was unconditionally
truncated to 0.

There was no live bug, since the only caller passes 0.

Fix, and back-patch to 14 where the function arrived.

Author: Justin Pryzby <pryzby@telsasoft.com>
Discussion: https://postgr.es/m/20230106031652.GR3109%40telsasoft.com

src/backend/storage/file/fd.c

@@ -638,7 +638,7 @@ pg_truncate(const char *path, off_t length)
 	fd = OpenTransientFile(path, O_RDWR | PG_BINARY);
 	if (fd >= 0)
 	{
-		ret = ftruncate(fd, 0);
+		ret = ftruncate(fd, length);
 		save_errno = errno;
 		CloseTransientFile(fd);
 		errno = save_errno;