rpc: drop inoperant newline escape sequence from wallet warning messages

in RPCs createwallet, loadwallet, and unloadwallet.

Even if the newline escape sequence was functioning rather than being printed,
it doesn't make sense to separate sentences inside a string field in the JSON
response with "\n".

Currently, only the createwallet RPC appends additional warning sentences and
is affected by this fix.

src/wallet/rpc/wallet.cpp

@@ -240,7 +240,7 @@ static RPCHelpMan loadwallet()
 
     UniValue obj(UniValue::VOBJ);
     obj.pushKV("name", wallet->GetName());
-    obj.pushKV("warning", Join(warnings, Untranslated("\n")).original);
+    obj.pushKV("warning", Join(/*container=*/warnings, /*separator=*/Untranslated(" ")).original);
 
     return obj;
 },
@@ -405,7 +405,7 @@ static RPCHelpMan createwallet()
 
     UniValue obj(UniValue::VOBJ);
     obj.pushKV("name", wallet->GetName());
-    obj.pushKV("warning", Join(warnings, Untranslated("\n")).original);
+    obj.pushKV("warning", Join(/*container=*/warnings, /*separator=*/Untranslated(" ")).original);
 
     return obj;
 },
@@ -464,7 +464,7 @@ static RPCHelpMan unloadwallet()
     UnloadWallet(std::move(wallet));
 
     UniValue result(UniValue::VOBJ);
-    result.pushKV("warning", Join(warnings, Untranslated("\n")).original);
+    result.pushKV("warning", Join(/*container=*/warnings, /*separator=*/Untranslated(" ")).original);
     return result;
 },
     };

test/functional/wallet_createwallet.py

@@ -17,7 +17,7 @@
 
 LEGACY_WALLET_MSG = "Wallet created successfully. The legacy wallet type is being deprecated and support for creating and opening legacy wallets will be removed in the future."
 EMPTY_PASSPHRASE_MSG = "Empty string given as passphrase, wallet will not be encrypted."
-EMPTY_PASSPHRASE_LEGACY_WALLET_MSG = f"{EMPTY_PASSPHRASE_MSG}\n{LEGACY_WALLET_MSG}"
+EMPTY_PASSPHRASE_LEGACY_WALLET_MSG = f"{EMPTY_PASSPHRASE_MSG} {LEGACY_WALLET_MSG}"
 
 
 class CreateWalletTest(BitcoinTestFramework):