Add earnings test for pension benefits

[MImmesberger]

What problem do you want to solve?

Until 2022, there was an earnings test for early retirees.

• If earnings are above some threshold, there is a 40% benefit deduction rate
• If earnings plus pension exceeds the highest labor market income of the last 15 years, there is a 100% deduction rate (Hinzuverdienstdeckel). For this, I added a new input variable höchster_bruttolohn_letzte_15_jahre_y.

This only applies for Vollrenten, we didn't implement Teilrenten yet.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 95.65217% with 2 lines in your changes missing coverage. Please review.

Project coverage is 89.88%. Comparing base (ffbccf3) to head (dc1793b). Report is 2 commits behind head on main.

Files	Patch %	Lines
src/_gettsim/shared.py	0.00%	1 Missing :warning:
src/_gettsim/transfers/unterhaltsvors.py	87.50%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #788 +/- ## ========================================== + Coverage 89.69% 89.88% +0.18% ========================================== Files 53 53 Lines 3796 3826 +30 ========================================== + Hits 3405 3439 +34 + Misses 391 387 -4 ```

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[MImmesberger]

While working on the comments I realized that the rule I implemented was partly introduced in 2017. Before that, there was a hard Hinzuverdienstgrenze above which pension benefits were set to 0. I'll implement the pre-2017 rule as well.

[MImmesberger]

Done!