Button Block: Remove backwards compatibility code for WP6.4

[t-hamano]

In #63082, backwards compatible code for WP6.4 was added to the Button block. This PR will be part of Gutenberg version 18.7.1.

This code should be reverted after the Gutenberg plugin drops support for WP6.4 in the near future.

gutenberg/packages/block-library/src/button/index.php

Lines 20 to 33 in 3687874

	/*
	* The current Gutenberg plugin supports WordPress 6.4, but the next_token()
	* method does not exist in WordPress 6.4. Therefore, if Gutenberg is used
	* as a plugin, use the Gutenberg class that has the `next_token()` method.
	*
	* TODO: After the Gutenberg plugin drops support for WordPress 6.4, this
	* conditional statement will be removed and the core class `WP_HTML_Tag_Processor`
	* should be used.
	*/
	if ( defined( 'IS_GUTENBERG_PLUGIN' ) && IS_GUTENBERG_PLUGIN && class_exists( 'Gutenberg_HTML_Tag_Processor_6_5' ) ) {
	$p = new Gutenberg_HTML_Tag_Processor_6_5( $content );
	} else {
	$p = new WP_HTML_Tag_Processor( $content );
	}

To make sure I don't forget to do this, I'll submit an issue.