Test whether `DEFAULT_TARGETS` can be computed with `create_synthetic_data` output

[MImmesberger]

Closes #733

I let the test run through the years 2017-2024. Extending further back does not work because the Unterhaltsvorschuss isn't implemented prior to 2017 (as discussed in #566).

I also added a NotImplementedError for the Unterhaltsvorschuss until 2016.

Related to #627

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.

Project coverage is 89.79%. Comparing base (ffbccf3) to head (56543f9). Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
src/_gettsim/transfers/unterhaltsvors.py	87.50%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #785 +/- ## ========================================== + Coverage 89.69% 89.79% +0.09% ========================================== Files 53 53 Lines 3796 3802 +6 ========================================== + Hits 3405 3414 +9 + Misses 391 388 -3 ```

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