Use DataBufferUtils.compose and remove writeAggregator

Use DataBufferUtils.compose instead of writeAggregator to combine
multiple data buffers into one, as the write aggregator would not work
when the initial data buffer did not have enough capacity to contain
all subsequent buffers.

Removed writeAggregator, as it is no longer needed.

Issue: SPR-16365

spring-core/src/test/java/org/springframework/core/io/buffer/DataBufferUtilsTests.java

@@ -308,21 +308,6 @@ public class DataBufferUtilsTests extends AbstractDataBufferAllocatingTestCase {
 		}
 	}
 
-	@Test
-	public void writeAggregator() {
-		DataBuffer foo = stringBuffer("foo");
-		DataBuffer bar = stringBuffer("bar");
-		DataBuffer baz = stringBuffer("baz");
-		Flux<DataBuffer> flux = Flux.just(foo, bar, baz);
-
-		DataBuffer result =
-				flux.reduce(DataBufferUtils.writeAggregator()).block(Duration.ofSeconds(1));
-
-		assertEquals("foobarbaz", DataBufferTestUtils.dumpString(result, StandardCharsets.UTF_8));
-
-		release(result);
-	}
-
 	@Test
 	public void SPR16070() throws Exception {
 		ReadableByteChannel channel = mock(ReadableByteChannel.class);