fix: align the declared PHP minimum with runtime APIs

[chubes4]

Current state

The original release-lint blockers were cleared in v0.24.2. The remaining concrete problem is runtime compatibility:

• extrachill-studio.php declares Requires PHP: 7.4.
• inc/transcription/callback.php uses str_ends_with(), introduced in PHP 8.0.

Decision required

Either:

1. Replace PHP 8-only calls with PHP 7.4-compatible equivalents and verify the whole plugin against 7.4, or
2. Raise the declared minimum consistently to the actual supported runtime.

The decision must follow the platform-wide PHP support policy rather than this plugin choosing independently.

Acceptance criteria

• The plugin contains no APIs newer than its declared PHP minimum.
• Homeboy compatibility, PHPCS, PHPStan, syntax, tests, and package validation pass.
• Plugin header, documentation, and release configuration agree on the minimum.
• No compatibility polyfill is added when a simple equivalent is sufficient.