Bump savegame version, copy town to city multipliers for old savegames

src/saveload/extended_ver_sl.cpp

@@ -148,6 +148,7 @@ const SlxiSubChunkInfo _sl_xv_sub_chunk_infos[] = {
 	{ XSLFI_INFLATION_FIXED_DATES,  XSCF_IGNORABLE_ALL,       1,   1, "inflation_fixed_dates",     nullptr, nullptr, nullptr        },
 	{ XSLFI_WATER_FLOODING,         XSCF_NULL,                1,   1, "water_flooding",            nullptr, nullptr, nullptr        },
 	{ XSLFI_MORE_HOUSES,            XSCF_NULL,                1,   1, "more_houses",               nullptr, nullptr, nullptr        },
+	{ XSLFI_CUSTOM_TOWN_ZONE,       XSCF_IGNORABLE_UNKNOWN,   1,   1, "custom_town_zone",          nullptr, nullptr, nullptr        },
 	{ XSLFI_NULL, XSCF_NULL, 0, 0, nullptr, nullptr, nullptr, nullptr },// This is the end marker
 };