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.

doc/release-notes-33819.md

@@ -0,0 +1,5 @@
+Mining IPC
+----------
+
+- The `getCoinbaseTx()` method is renamed to `getCoinbaseRawTx()`.
+  IPC clients do not use the function name, so they're not affected. (#33819)