Revert phpcs back to PHP 5.6 (#52384)

Reverts phpcs PHP compatibility version back to 5.6.

Author: hellofromtonya

phpcs.xml.dist

@@ -2,7 +2,7 @@
 <ruleset name="WordPress Coding Standards for Gutenberg Plugin">
 	<description>Sniffs for WordPress plugins, with minor modifications for Gutenberg</description>
 
-	<config name="testVersion" value="7.0-"/>
+	<config name="testVersion" value="5.6-"/>
 	<rule ref="PHPCompatibilityWP">
 		<include-pattern>*\.php$</include-pattern>
 	</rule>