Merge bitcoin/bitcoin#34197: rpc, net: deprecate startingheight field of getpeerinfo RPC

4ce3f4a265 rpc, net: deprecate `startingheight` field of `getpeerinfo` RPC (Sebastian Falbesoner)

Pull request description:

  This PR deprecates the "startingheight" result field of the `getpeerinfo` RPC, following the discussion in #33990.

  Rationale: 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 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.

ACKs for top commit:
  optout21:
    crACK 4ce3f4a265
  achow101:
    ACK 4ce3f4a265
  fjahr:
    utACK 4ce3f4a265
  rkrux:
    crACK 4ce3f4a265
  janb84:
    cr ACK 4ce3f4a265

Tree-SHA512: b296a28d30084fd35c67a2162e85576e3365e5d6fffe5b1add500034c1850604ee8c37b61afe812bfab8a7cc20f6a9e22db445e3c371311a5f82a777e5700ebf

test/functional/rpc_net.py

@@ -62,7 +62,10 @@ def seed_addrman(node):
 class NetTest(BitcoinTestFramework):
     def set_test_params(self):
         self.num_nodes = 2
-        self.extra_args = [["-minrelaytxfee=0.00001000"], ["-minrelaytxfee=0.00000500"]]
+        self.extra_args = [
+            ["-minrelaytxfee=0.00001000", "-deprecatedrpc=startingheight"],
+            ["-minrelaytxfee=0.00000500"],
+        ]
         # Specify a non-working proxy to make sure no actual connections to public IPs are attempted
         for args in self.extra_args:
             args.append("-proxy=127.0.0.1:1")