CI: Update diffrax branch

[dweindl]

dev is gone:
https://github.com/patrick-kidger/diffrax/branches/all?query=dev

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.56%. Comparing base (9ede529) to head (af8ae1f).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #3123       +/-   ##
===========================================
+ Coverage   44.66%   77.56%   +32.90%     
===========================================
  Files         111      315      +204     
  Lines       17084    20482     +3398     
  Branches     1417     1484       +67     
===========================================
+ Hits         7630    15887     +8257     
+ Misses       9128     4587     -4541     
+ Partials      326        8      -318     

Flag	Coverage Δ
cpp	71.69% <ø> (?)
cpp_python	37.85% <ø> (?)
petab	46.56% <ø> (+10.84%)	⬆️
petab_sciml	13.93% <ø> (ø)
python	70.35% <ø> (+16.85%)	⬆️
sbmlsuite-jax	?

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see 289 files with indirect coverage changes

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