MDL-79674 behat: Bump axe-core to v4.8.4

* Improve readme_moodle.txt and add more detailed instructions
* Update \behat_accessibility::run_axe_validation_for_tags()'s
  PHPDoc block to reflect the current version.
* Fix incorrect copyright tag for the library

lib/behat/axe/readme_moodle.txt

@@ -2,16 +2,31 @@ Description of axe import into Moodle
 
 1/ Download the latest axe code somewhere (example /tmp/axe) using:
 
-mkdir -p axe/node_modules
-npm install --prefix axe axe-core --save-dev
+mkdir -p /tmp/axe
+cd /tmp/axe
+npm install axe-core --save-dev
 
 Note down the version number displayed by the command, to update lib/thirdpartylibs.xml accordingly.
 
+If the command does not output a version number, the version number can be found in package.json. You can simply open package.json
+using your desired editor and look for the version number of axe-core. Alternatively, you may use the following commands:
+- MacOS:
+    cat package.json | grep axe-core
+- Linux:
+    cat package.json | grep '"version"' | awk -F '"' '{print $4}'
+    or
+    jq -r '.version' package.json
+
 2/ Copy the following file to your local Moodle directory, to replace the old one:
 
-cp axe/node_modules/axe-core/axe.min.js [PATH TO YOUR MOODLE]/lib/behat/axe/
+cp /tmp/axe/node_modules/axe-core/axe.min.js [PATH TO YOUR MOODLE]/lib/behat/axe/
+
+3/ Update lib/thirdpartylibs.xml with the new version number.
+
+4/ Update the PHPDoc block of \behat_accessibility::run_axe_validation_for_tags() in
+[PATH TO YOUR MOODLE]/lib/tests/behat/behat_accessibility.php to reflect the new version number.
 
-3/ Run behat tests labelled with @accessibility and confirm they are passing with the new library version, or fix the failures
+5/ Run behat tests labelled with @accessibility and confirm they are passing with the new library version, or fix the failures
 because the new version might raise issues that weren't detected previously:
 
 php admin/tool/behat/cli/init.php --add-core-features-to-theme

lib/tests/behat/behat_accessibility.php

@@ -46,7 +46,7 @@ class behat_accessibility extends behat_base {
      * It is also possible to specify any desired optional tags.
      *
      * The list of available tags can be found at
-     * https://github.com/dequelabs/axe-core/blob/v3.5.5/doc/rule-descriptions.md.
+     * https://github.com/dequelabs/axe-core/blob/v4.8.4/doc/rule-descriptions.md.
      *
      * @Then the page should meet accessibility standards
      * @Then the page should meet accessibility standards with :extratags extra tests

lib/thirdpartylibs.xml

@@ -17,12 +17,12 @@
     <location>behat/axe</location>
     <name>axe-core</name>
     <description>Accessibility testing engine for websites and other HTML-based user interfaces.</description>
-    <version>4.7.2</version>
+    <version>4.8.4</version>
     <license>MPL</license>
     <licenseversion>2.0</licenseversion>
     <repository>https://github.com/dequelabs/axe-core</repository>
     <copyrights>
-      <copyright>2005, Jon Papaioannou</copyright>
+      <copyright>Deque Systems, Inc.</copyright>
     </copyrights>
   </library>
   <library>