Fix: [Script] ScriptSubsidy::GetExpireDate should return an economy-date

[frosch123]

Motivation / Problem

As discussed in #12339:

Currently ScriptSubsidy::GetExpireDate adds economy-months to ScriptDate::GetDate.
If ScriptTimeMode is set to CalendarTime, this will add economy-months to a calendar-date, which makes no sense.

Description

ScriptSubsidy::GetExpireDate should always return an economy-date.

Limitations

Checklist for review

Some things are not automated, and forgotten often. This list is a reminder for the reviewers.

• The bug fix is important enough to be backported? (label: 'backport requested')
• This PR touches english.txt or translations? Check the guidelines
• This PR affects the save game format? (label 'savegame upgrade')
• This PR affects the GS/AI API? (label 'needs review: Script API')
  • ai_changelog.hpp, game_changelog.hpp need updating.
  • The compatibility wrappers (compat_*.nut) need updating.
• This PR affects the NewGRF API? (label 'needs review: NewGRF')
  • newgrf_debug_data.h may need updating.
  • PR must be added to API tracker