rpc, net: deprecate startingheight field of getpeerinfo RPC

The reported starting height of a peer in the VERSION message is
untrusted, and it doesn't seem to be useful anymore (after #20624),
so deprecating the corresponding "startingheight" field seems
reasonable. After that, it can be removed, along with the
`m_starting_height` field of the Peer / CNodeStats structs, as it is
sufficient to show the reported height only once at connection in the
debug log.

doc/release-notes-34197.md

@@ -0,0 +1,7 @@
+Updated RPCs
+------------
+
+- The `getpeerinfo` RPC no longer returns the `startingheight` field unless
+  the configuration option `-deprecatedrpc=startingheight` is used. The
+  `startingheight` field will be fully removed in the next major release.
+  (#34197)