sources/oauth: fix apple authentication

[Rsgm]

Details

Fixes the previously removed post body authentication scheme that breaks Apple authentication.
Apple expects client credentials in the token post body, instead of basic auth.
This attempts to maintain the fix in the previous PR while addressing the new issue affecting the Apple source.

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Checklist

• Local tests pass (ak test authentik/)
• The code has been formatted (make lint-fix)

If an API change has been made

• The API schema has been updated (make gen-build)

If changes to the frontend have been made

• The code has been formatted (make web)

If applicable

• The documentation has been updated
• The documentation has been formatted (make website)

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[codecov]

Codecov Report

Attention: Patch coverage is 84.61538% with 2 lines in your changes missing coverage. Please review.

Project coverage is 92.73%. Comparing base (41dc23b) to head (8fc0860).
Report is 223 commits behind head on main.

Files with missing lines	Patch %	Lines
authentik/sources/oauth/clients/oauth2.py	81.81%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #13958      +/-   ##
==========================================
- Coverage   92.78%   92.73%   -0.06%     
==========================================
  Files         796      796              
  Lines       40761    40767       +6     
==========================================
- Hits        37821    37804      -17     
- Misses       2940     2963      +23     

Flag	Coverage Δ
e2e	47.79% <84.61%> (-0.08%)	⬇️
integration	24.17% <53.84%> (+<0.01%)	⬆️
unit	90.54% <53.84%> (-0.02%)	⬇️

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