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.

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")