In the spenden (donation) table, we have two export timestamps (dt_gruen, dt_exp) while only one is actually needed.

Acceptance Criteria:

• spenden contains only an export_date field.
• Entities use the new export_date field for isExported and as accessors.
• The export_date field is mapped to dt_gruen in the export (if that field still exists in the export)
• Depending on the outcome of T238208: Investigate performance impact of nullable export date column, create a new indexed boolean column that marks the exported state and remove the index on the export date.