Add graphql-dataloader: Spring GraphQL 2.0 DataLoader batching vs naive N+1, and non-null propagation on a dangling FK

- naive @SchemaMapping resolver: 6 statements (5 books/5 authors), 21 statements (20 books/5 authors)
- batched @BatchMapping resolver: flat 2 statements in both cases, via DataLoader + .distinct()
- dangling authorId nulls the entire GraphQL response via non-null propagation, byte-identical under both resolver strategies
- 6-test suite over real HTTP against a live embedded Tomcat instance, SQL captured via a JDK dynamic proxy (StatementLoggingDataSource, reused from sdjpa4-demo)
- docs/05: two Boot 4.1 packaging changes hit along the way (DataSourceAutoConfiguration's new package, Jackson 3 by default)
- root README: add row for graphql-dataloader; fix openapi-versioning's placeholder link now that post 7477 is live

graphql-dataloader/docs/output/batched-c-dangling-foreign-key-null-propagation.txt

@@ -0,0 +1,4 @@
+batched profile / one Book has authorId=9999999 which matches no Author row
+raw HTTP response body from POST /graphql:
+
+{"errors":[{"message":"The field at path '/books[1]/author' was declared as a non null type, but the code involved in retrieving data has wrongly returned a null value.  The graphql specification requires that the parent field be set to null, or if that is non nullable that it bubble up null to its parent and so on. The non-nullable type is 'Author' within parent type 'Book'","path":["books",1,"author"],"extensions":{"classification":"NullValueInNonNullableField"}}],"data":null}