Remove deprecated methods to User::class.

[terabytesoftw]

Q	A
Is bugfix?	❌
New feature?	✔️
Breaks BC?	❌

[what-the-diff]

PR Summary

• Removal of Deprecated getAuthManager() method in User.php
  The older getAuthManager() method, which is due to be removed in an upcoming version (2.1), has been eliminated from the User.php file. Its functionality has become obsolete due to new changes.
• Update to getAccessChecker() method in User.php
  The getAccessChecker() method has been updated to return the value of Yii::$app->getAuthManager() when the accessChecker property has no assigned value, effectively replacing the functionality of the now-removed getAuthManager() method. This adds stability and ensures the system doesn't experience failures due to lack of an assigned value.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +39.06% 🎉

Comparison is base (6618aa8) 28.12% compared to head (12591df) 67.18%.

Additional details and impacted files

@@              Coverage Diff              @@
##                2.2   #19972       +/-   ##
=============================================
+ Coverage     28.12%   67.18%   +39.06%     
+ Complexity    11209    11208        -1     
=============================================
  Files           424      424               
  Lines         35617    35615        -2     
=============================================
+ Hits          10016    23928    +13912     
+ Misses        25601    11687    -13914     

Files Changed	Coverage Δ
framework/web/User.php	87.17% <100.00%> (+87.17%)	⬆️

... and 203 files with indirect coverage changes

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