Add note about removed methods for v8

Author: luke-hill

upgrading_notes/8.0.0.md

@@ -121,3 +121,6 @@ end
 table.map_column('column', strict: false) do |value|
 end
 ```
+
+NB: Both the `#map_column!` variant method and `#map_headers!` sibling method have been removed (as per
+the changelog), you should use the non-mutative alternative methods `#map_column` and `#map_headers` instead