56025: update wp_valida_boolean() to work with truthy strings

[CallumBW95]

Update the wp_validate_boolean() function to work with truthy values.

Trac ticket: #56025

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[github-actions]

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Props callumbw95, mindctrl.

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[github-actions]

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[mindctrl]

@CallumBW95 this PR causes existing unit tests to fail.

1) Tests_Functions_wpValidateBoolean::test_wp_validate_boolean with data set #4 ('false', false)
Failed asserting that true is identical to false.

/var/www/tests/phpunit/tests/functions/wpValidateBoolean.php:24

2) Tests_Functions_wpValidateBoolean::test_wp_validate_boolean with data set #5 ('FalSE', false)
Failed asserting that true is identical to false.

/var/www/tests/phpunit/tests/functions/wpValidateBoolean.php:24

3) Tests_Functions_wpValidateBoolean::test_wp_validate_boolean with data set #6 ('FALSE', false)
Failed asserting that true is identical to false.

/var/www/tests/phpunit/tests/functions/wpValidateBoolean.php:24