fix: use regional terrain for s_garage_2 and other changes

[0Monet]

Checklist

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Purpose of change

Some changes to the mapgen s_garage_2 such as using regional terrain, removing unnecessary symbols and alias.

Describe the solution

Replace non-regional outdoor terrain with pseudo-regional terrain.
Replace t_grate with t_grate_indoor

Describe alternatives you've considered

none

Additional context

Before:

After: