[6.0] Move CMS Filesystem package to compat plugin (joomla#44240)

Thanks @Hackwar, and Phil for testing.

Author: Hackwar

administrator/language/en-GB/plg_behaviour_compat.ini

@@ -8,6 +8,8 @@ PLG_COMPAT_FIELD_CLASSES_ALIASES_LABEL="Classes Aliases"
 PLG_COMPAT_FIELD_CLASSES_ALIASES_DESCRIPTION="Add class aliases for classes which have been renamed or moved to a namespace."
 PLG_COMPAT_FIELD_ES5_ASSETS_DESCRIPTION="Activate this option if your extension requires *.es5 assets which has resulted in an exception. The assets provided are empty but prevent the exception."
 PLG_COMPAT_FIELD_ES5_ASSETS_LABEL="ES5 Assets"
+PLG_COMPAT_FIELD_LEGACY_CLASSES_DESCRIPTION="Activate this option if your extension requires classes deprecated for the current version of Joomla. This includes those removed classes from the folder /plugins/behavior/compat/classes/."
+PLG_COMPAT_FIELD_LEGACY_CLASSES_LABEL="Include Deprecated Classes"
 PLG_COMPAT_FIELD_REMOVED_ASSETS_DESCRIPTION="Activate this option if your extension requires removed assets which has resulted in an exception. The assets provided are empty but prevent the exception."
 PLG_COMPAT_FIELD_REMOVED_ASSETS_LABEL="Removed Assets"
 PLG_COMPAT_XML_DESCRIPTION="If you use extensions which are not using the current Joomla Coding standards then this plugin, when enabled, will provide backward compatibility to the prior major version."