chore: bump app router example to use nextjs 15

[markphelps]

To try out nextJS 15 app router. It still works

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 63.63%. Comparing base (28dbe11) to head (e8ace50).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3997      +/-   ##
==========================================
- Coverage   63.64%   63.63%   -0.01%     
==========================================
  Files         170      170              
  Lines       17360    17360              
==========================================
- Hits        11048    11047       -1     
- Misses       5646     5647       +1     
  Partials      666      666              

Flag	Coverage Δ
unittests	63.63% <ø> (-0.01%)	⬇️

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