Merge #21404: refactor: Remove MakeUnique<T>()

1a6323bdbe doc: update developer notes for removal of MakeUnique (fanquake)
3ba2840e7e scripted-diff: remove MakeUnique<T>() (fanquake)

Pull request description:

  Since requiring C++17, this is just pointless abstraction. I think we should just "tear the band-aid off" and remove it. Similar to the changes happening in #21366.

  Also, having a comment saying this is deprecated doesn't prevent it's usage in new code. i.e : https://github.com/bitcoin/bitcoin/pull/20946#discussion_r561949731.

  The repository is fairly quiet at the moment, so any potential complaints about having to rebase should be minimal. Might as well get this over and done with.

ACKs for top commit:
  jnewbery:
    utACK 1a6323bdbe
  practicalswift:
    cr ACK 1a6323bdbe: patch looks correct
  ajtowns:
    ACK 1a6323bdbe -- code review only
  glozow:
    ACK https://github.com/bitcoin/bitcoin/commit/1a6323bdbe20bdb7b1c907d8fa0333ffa88b21ff looks correct

Tree-SHA512: 4a14b9611b60b9b3026b54d6f5a2dce4c5d9b63a7b93d7de1307512df736503ed84bac66e7b93372c76e3117f49bf9f29cd473d3a47cb41fb2775bc10234736f

src/qt/sendcoinsdialog.cpp

@@ -266,7 +266,7 @@ bool SendCoinsDialog::PrepareSendText(QString& question_string, QString& informa
     }
 
     // prepare transaction for getting txFee earlier
-    m_current_transaction = MakeUnique<WalletModelTransaction>(recipients);
+    m_current_transaction = std::make_unique<WalletModelTransaction>(recipients);
     WalletModel::SendCoinsReturn prepareStatus;
 
     updateCoinControlState(*m_coin_control);