Fix phpstan notice on latest phpstan

[samsonasik]

noticed on:

https://github.com/rectorphp/rector-src/actions/runs/25209017650/job/73915428002?pr=7975#step:7:15

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  rules/DowngradePhp72/Rector/ClassMethod/DowngradeParameterTypeWideningRector.php:147
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  - '#Property Rector\\DowngradePhp72\\Rector\\ClassMethod\\DowngradeParameterTypeWideningRector\:\:\$unsafeTypesToMethods \(array<string, array<string>>\) does not accept array<string, iterable<string>>#'
  🪪 assign.propertyType
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