[DOCS] System indices no longer accessible 8.0 (#84377) (#84438)

We know that we plan to remove direct access to system indices, but we aren't sure what major version that change will fall in. This updates the docs to avoid any confusion in the meantime.

Co-authored-by: James Rodewig <40268737+jrodewig@users.noreply.github.com>
(cherry picked from commit ffd21e52594edaa0b6eabce8398e524ece371a5e)

Co-authored-by: Stef Nestor <steffanie.nestor@gmail.com>

docs/reference/api-conventions.asciidoc

@@ -245,7 +245,7 @@ You should not directly access or modify system indices
 as they contain data essential to the operation of the system.
 
 IMPORTANT: Direct access to system indices is deprecated and
-will no longer be allowed in the next major version.
+will no longer be allowed in a future major version.
 
 [discrete]
 [[api-conventions-parameters]]