[AVRO-3942] Mark MemoryOutputStream as final (#2752)

Otherwise the class raises a compilation warning:

lang/c++/impl/Stream.cc:129:27: warning: class with destructor marked 'final' cannot be inherited from [-Wfinal-dtor-non-final-class]
  120 | class MemoryOutputStream : public OutputStream {
      |                          final
  121 | public:
  122 |     const size_t chunkSize_;
  123 |     std::vector<uint8_t *> data_;
  124 |     size_t available_;
  125 |     size_t byteCount_;
  126 |
  127 |     explicit MemoryOutputStream(size_t chunkSize) : chunkSize_(chunkSize),
  128 |                                                     available_(0), byteCount_(0) {}
  129 |     ~MemoryOutputStream() final {
      |                           ^
lang/c++/impl/Stream.cc:120:7: note: mark 'avro::MemoryOutputStream' as 'final' to silence this warning
  120 | class MemoryOutputStream : public OutputStream {
      |       ^

lang/c++/impl/Stream.cc

@@ -117,7 +117,7 @@ class MemoryInputStream2 : public InputStream {
     }
 };
 
-class MemoryOutputStream : public OutputStream {
+class MemoryOutputStream final : public OutputStream {
 public:
     const size_t chunkSize_;
     std::vector<uint8_t *> data_;