fix #4511 & #5137: when adding a child to a family, put it between older and younger children

[BertKoor]

Inspired on the algorithm of the 1.7 branch: put a new child after older children, just before the first child which is younger.

This saves the user from having to reorder or sort children after a child is added.

I did try to make some actual tests, avoid mocking everything, but alas got stuck on tree authorisations when creating a family or child.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 21 lines in your changes missing coverage. Please review.
✅ Project coverage is 34.95%. Comparing base (083a110) to head (e470e96).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
app/Services/FamilyService.php	0.00%	15 Missing ⚠️
...pp/Http/RequestHandlers/AddChildToFamilyAction.php	0.00%	3 Missing ⚠️
...p/Http/RequestHandlers/LinkChildToFamilyAction.php	0.00%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #5260      +/-   ##
============================================
- Coverage     34.96%   34.95%   -0.02%     
- Complexity    11226    11233       +7     
============================================
  Files          1159     1160       +1     
  Lines         48238    48256      +18     
============================================
  Hits          16867    16867              
- Misses        31371    31389      +18     

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