MDL-79674 behat: Disable target-size rule

Since axe-core has introduced WCAG 2.2 Level AA support, a `target-size`
rule has been added which is a Level AA success criterion. Since we
haven't started working on WCAG 2.2 Level AA compliance, let's disable
this rule for now.

See https://www.w3.org/WAI/WCAG22/Understanding/target-size-minimum.html

lib/tests/behat/behat_accessibility.php

@@ -218,6 +218,12 @@ EOF;
                 'type' > 'tag',
                 'values' => array_merge($standardtags, $extratags),
             ],
+            // TODO: MDL-80831 - Disable the target-size rule for now until we start working towards WCAG 2.2 Level AA compliance.
+            'rules' => [
+                'target-size' => [
+                    'enabled' => false,
+                ],
+            ],
         ]);
     }
 }