rpc: deprecate "warning" field in {create,load,unload,restore}wallet

This string field has been replaced in these four RPCs by a "warnings" field
returning a JSON array of strings.

test/functional/wallet_backwards_compatibility.py

@@ -265,7 +265,12 @@ class BackwardsCompatibilityTest(BitcoinTestFramework):
             )
             load_res = node_master.loadwallet("u1_v16")
             # Make sure this wallet opens without warnings. See https://github.com/bitcoin/bitcoin/pull/19054
-            assert_equal(load_res['warning'], '')
+            if int(node_master.getnetworkinfo()["version"]) >= 249900:
+                # loadwallet#warnings (added in v25) -- only present if there is a warning
+                assert "warnings" not in load_res
+            else:
+                # loadwallet#warning (deprecated in v25) -- always present, but empty string if no warning
+                assert_equal(load_res["warning"], '')
             wallet = node_master.get_wallet_rpc("u1_v16")
             info = wallet.getaddressinfo(v16_addr)
             descriptor = f"wpkh([{info['hdmasterfingerprint']}{hdkeypath[1:]}]{v16_pubkey})"

test/functional/wallet_createwallet.py

@@ -25,6 +25,7 @@ class CreateWalletTest(BitcoinTestFramework):
 
     def set_test_params(self):
         self.num_nodes = 1
+        self.extra_args = [["-deprecatedrpc=walletwarningfield"]]
 
     def skip_test_if_missing_module(self):
         self.skip_if_no_wallet()