MINOR: Fix GetReadableBuffer compilation with gRPC 1.79.0

Replace ReadableBuffer.readBytes(ByteBuffer) with readBytes(byte[], int, int)
as the ByteBuffer overload was removed in gRPC 1.79.0.

Author: jbonofre

flight/flight-core/src/main/java/org/apache/arrow/flight/grpc/GetReadableBuffer.java

@@ -87,13 +87,13 @@ public static void readIntoBuffer(
       final InputStream stream, final ArrowBuf buf, final int size, final boolean fastPath)
       throws IOException {
     ReadableBuffer readableBuffer = fastPath ? getReadableBuffer(stream) : null;
+    byte[] heapBytes = new byte[size];
     if (readableBuffer != null) {
-      readableBuffer.readBytes(buf.nioBuffer(0, size));
+      readableBuffer.readBytes(heapBytes, 0, size);
     } else {
-      byte[] heapBytes = new byte[size];
       ByteStreams.readFully(stream, heapBytes);
-      buf.writeBytes(heapBytes);
     }
+    buf.writeBytes(heapBytes);
     buf.writerIndex(size);
   }
 }