[DOCS] Add 7.8 release notes entry for auto create index change (#57582)

The create index action name (`indices:admin/create`) can no longer be used to grant privileges to auto create indices and instead the `create_index` builtin privilege should be used.

Relates to #55858

Co-authored-by: Jake Landis <jake.landis@elastic.co>

docs/reference/release-notes/7.8.asciidoc

@@ -12,3 +12,7 @@ Search::
   a numeric field and a `GeoPoint` if they are counting a `geo_point` fields.
   They used to always receive the `String` representation of those values.
   {pull}54854[#54854]
+
+Features/Indices APIs::
+*  The privilege named `indices:admin/create` will no longer allow the auto 
+   creation of indices.  Use `create_index` instead. {pull}55858[#55858]