Change: Use per-company group numbers. (#12297)

This is used by the default group name, replacing the use of group index.

src/saveload/saveload.h

@@ -380,6 +380,7 @@ enum SaveLoadVersion : uint16_t {
 	SLV_VEHICLE_ECONOMY_AGE,                ///< 334  PR#12141 v14.0 Add vehicle age in economy year, for profit stats minimum age
 
 	SLV_COMPANY_ALLOW_LIST,                 ///< 335  PR#12337 Saving of list of client keys that are allowed to join this company.
+	SLV_GROUP_NUMBERS,                      ///< 336  PR#12297 Add per-company group numbers.
 
 	SL_MAX_VERSION,                         ///< Highest possible saveload version
 };