mining: rename getCoinbaseTx() to ..RawTx()

This frees up the name getCoinbaseTx() for the next commit.

Changing a function name does not impact IPC clients, as they only
consider the function signature and sequence number.

src/node/interfaces.cpp

@@ -888,7 +888,7 @@ public:
         return m_block_template->vTxSigOpsCost;
     }
 
-    CTransactionRef getCoinbaseTx() override
+    CTransactionRef getCoinbaseRawTx() override
     {
         return m_block_template->block.vtx[0];
     }