Fix #10334: Store separate newgrf-safe version of date_of_last_service. (#11124)

This value is not changed when the date cheat is used, which caused issues with changing properties based on service date.

Co-authored-by: Peter Nelson <peter1138@openttd.org>
(cherry picked from commit 9a602ff304)

src/sl/extended_ver_sl.cpp

@@ -201,6 +201,7 @@ const SlxiSubChunkInfo _sl_xv_sub_chunk_infos[] = {
 	{ XSLFI_EXTEND_VEHICLE_RANDOM,            XSCF_NULL,                1,   1, "slv_extend_vehicle_random",        nullptr, nullptr, nullptr          },
 	{ XSLFI_DISASTER_VEH_STATE,               XSCF_NULL,                1,   1, "slv_disaster_veh_state",           nullptr, nullptr, nullptr          },
 	{ XSLFI_SAVEGAME_ID,                      XSCF_NULL,                1,   1, "slv_savegame_id",                  nullptr, nullptr, nullptr          },
+	{ XSLFI_NEWGRF_LAST_SERVICE,              XSCF_NULL,                1,   1, "slv_newgrf_last_service",          nullptr, nullptr, nullptr          },
 	{ XSLFI_NULL, XSCF_NULL, 0, 0, nullptr, nullptr, nullptr, nullptr },// This is the end marker
 };