Formatting: Skip readonly properties in map_deep() to prevent fatal errors on PHP 8.1+

[himanshupathak95]

Trac ticket: https://core.trac.wordpress.org/ticket/60355

On PHP 8.1+, passing an object with readonly properties through map_deep() causes a fatal error because the function attempts to reassign every property after applying the callback.

This affects any code path that stores objects in metadata, since update_metadata() → wp_unslash() → stripslashes_deep() calls map_deep() internally.

This PR adds a PHP_VERSION_ID >= 80100 guard inside map_deep() that uses ReflectionProperty::isReadOnly() to detect and skip readonly properties.

WIP: Will be adding unit tests for this shortly.

[github-actions]

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