Drop Composer v1 support

[retlehs]

Composer v1 has been EOL since October 2022.

• Drop Composer v1 support (composer-plugin-api and composer/composer constraints updated to ^2.0)
• Remove v1/v2 version-switching logic in WordPressCorePluginTest
• Fix PHPUnit deprecation warning by replacing expectExceptionMessageRegExp with expectExceptionMessageMatches
• Pin CI to Composer v2
• Add Requirements section to README

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.74%. Comparing base (adc6ac2) to head (7aa68ee).
⚠️ Report is 1 commits behind head on master.
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Additional details and impacted files

@@            Coverage Diff            @@
##             master      #10   +/-   ##
=========================================
  Coverage     89.74%   89.74%           
  Complexity       20       20           
=========================================
  Files             2        2           
  Lines            39       39           
=========================================
  Hits             35       35           
  Misses            4        4           

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