Fix: remove incompatible return type

[saulens22]

Fixes a bug introduced in #2506

Commit: 226a709

setAttribute in Laravel has it's return type declared as mixed, making this method incompatible: https://github.com/laravel/framework/blob/10.x/src/Illuminate/Database/Eloquent/Concerns/HasAttributes.php#L933-L940

This pull request removes return type. A method with no return type is mixed by default.

[codecov-commenter]

Codecov Report

Base: 89.91% // Head: 89.91% // No change to project coverage 👍

Coverage data is based on head (7b814df) compared to base (57010c0).
Patch coverage: 100.00% of modified lines in pull request are covered.

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Additional details and impacted files

@@            Coverage Diff            @@
##             master    #2517   +/-   ##
=========================================
  Coverage     89.91%   89.91%           
  Complexity      712      712           
=========================================
  Files            32       32           
  Lines          1726     1726           
=========================================
  Hits           1552     1552           
  Misses          174      174           

Impacted Files	Coverage Δ
src/Eloquent/Model.php	91.66% <100.00%> (ø)

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