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/src/main/resources/schema.sql

@@ -0,0 +1,10 @@
+create table author (
+    id bigint auto_increment primary key,
+    name varchar(200) not null
+);
+
+create table book (
+    id bigint auto_increment primary key,
+    title varchar(200) not null,
+    author_id bigint
+);