fix: replace deprectated property

[cbscsm]

• Litestar.static_files_config property is deprecated since 2.6.0 by feat: Implement static file serving with regular route handlers #2960
• But Litestar.__init__ uses the property. I'm not sure that it is intended or not, I can't avoid that deprecated warning from calling Litestar(...).

  DeprecationWarning: Use of deprecated property 'static_files_config'. Deprecated in litestar 2.6.0. This property will be removed in the next major version. Use create_static_files router instead
  

• I suggest replace the property access to internal attribute in internal code.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (1aee828) 98.27% compared to head (fb297a7) 98.27%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3087   +/-   ##
=======================================
  Coverage   98.27%   98.27%           
=======================================
  Files         320      320           
  Lines       14353    14353           
  Branches     2306     2306           
=======================================
  Hits        14106    14106           
  Misses        107      107           
  Partials      140      140           

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[guacs]

@cbscsm the failing build should be fixed by #3089.

[github-actions]

Documentation preview will be available shortly at https://litestar-org.github.io/litestar-docs-preview/3087