Use symfony/polyfill-php80

[nanasess]

Fixes #816

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以下 Copilot actions のコメント

This pull request includes several changes to improve compatibility and clean up deprecated code. The most important changes include adding a new dependency to composer.json, removing deprecated PHP compatibility functions, and updating the PHPStan configuration.

Dependency updates:

• composer.json: Added symfony/polyfill-php80 to ensure compatibility with PHP 8.0. (composer.jsonL50-R51)

Code cleanup:

• data/class/SC_Initial.php: Removed the compatPhp method and its call in the init method, as it contained deprecated PHP compatibility functions that are no longer needed. ([1], [2])

Configuration updates:

• phpstan.neon.dist: Removed an unnecessary PHPStan error ignore rule related to inner named functions, as it is no longer applicable. ([phpstan.neon.distL41-L43](https://github.com/EC-CUBE/ec-cube2/pull/1042/files#diff-6f19df6a6307a48db0940e6897591fb08776d2db8a6134737aa708defdb5c92dL41-L43))

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 51.76%. Comparing base (cdefe2a) to head (d685168).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1042      +/-   ##
==========================================
+ Coverage   51.67%   51.76%   +0.08%     
==========================================
  Files          80       80              
  Lines       10157    10195      +38     
==========================================
+ Hits         5249     5277      +28     
- Misses       4908     4918      +10     

Flag	Coverage Δ
tests	51.76% <ø> (+0.08%)	⬆️

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