KSES: Use the polyfilled PHP 8 string functions in _wp_kses_allow_pdf_objects():

* `str_contains()`
* `str_ends_with()`
* `str_starts_with()`

Additionally, include a test for a PDF file in an `<object>` tag with an unsupported protocol.

Follow-up to [51963], [52039], [52040], [52304], [52309].

Props TobiasBg, ramonopoly.
See #54261.

git-svn-id: https://develop.svn.wordpress.org/trunk@52326 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/tests/kses.php

@@ -1588,10 +1588,14 @@ EOF;
 				'<object type="application/pdf" data="https://example.org/foo.php" />',
 				'',
 			),
-			'protocol relative url'                   => array(
+			'protocol-relative url'                   => array(
 				'<object type="application/pdf" data="//example.org/foo.pdf" />',
 				'',
 			),
+			'unsupported protocol'                    => array(
+				'<object type="application/pdf" data="ftp://example.org/foo.pdf" />',
+				'',
+			),
 			'relative url'                            => array(
 				'<object type="application/pdf" data="/cat/foo.pdf" />',
 				'',