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]

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 ⚠️

Additional details and impacted files

@@            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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[hmgaudecker]

Great, thanks!