Add bold off SGR

st.c

@@ -1402,6 +1402,9 @@ tsetattr(int *attr, int l)
 		case 9:
 			term.c.attr.mode |= ATTR_STRUCK;
 			break;
+		case 21:
+			term.c.attr.mode &= ~ATTR_BOLD;
+			break;
 		case 22:
 			term.c.attr.mode &= ~(ATTR_BOLD | ATTR_FAINT);
 			break;