[7.8] remove the term 'system' from indicies doc (#56367) (#56374)

'system' indices will carry special meaning in the future this commit
removes the system from the name to avoid confusion. (technically
these indices will be hidden not system)

docs/reference/setup/install/xpack-indices.asciidoc

@@ -1,4 +1,4 @@
-Some commercial features automatically create system indices within {es}.
+Some commercial features automatically create indices within {es}.
 By default, {es} is configured to allow automatic index creation, and no
 additional steps are required. However, if you have disabled automatic index
 creation in {es}, you must configure