Docs: Add note about removing prepareExecute from the java client

relates elastic#30966

Author: rjernst

docs/reference/migration/migrate_7_0/java.asciidoc

@@ -5,4 +5,10 @@
 
 `isShardsAcked` has been replaced by `isShardsAcknowledged` in
 `CreateIndexResponse`, `RolloverResponse` and
-`CreateIndexClusterStateUpdateResponse`.
+`CreateIndexClusterStateUpdateResponse`.
+
+==== `prepareExecute` removed from the client api
+
+The `prepareExecute` method which created a request builder has been
+removed from the client api. Instead, construct a build for the
+appropriate request directly.