wallet: Set upgraded descriptor cache flag for newly created wallets

Although WalletBatch::LoadWallet performs the descriptor cache upgrade,
because new wallets do not have the descriptor flag set yet, the upgrade
does not run and set the flag.

Since new wallets will always being using the upgraded cache, there's no
reason to wait to set the flag, so set it when the wallet flags are
being initialized for new wallets.

Author: achow101

src/wallet/wallet.cpp

@@ -2895,7 +2895,9 @@ std::shared_ptr<CWallet> CWallet::Create(WalletContext& context, const std::stri
         // ensure this wallet.dat can only be opened by clients supporting HD with chain split and expects no default key
         walletInstance->SetMinVersion(FEATURE_LATEST);
 
-        walletInstance->InitWalletFlags(wallet_creation_flags);
+        // Init with passed flags.
+        // Always set the cache upgrade flag as this feature is supported from the beginning.
+        walletInstance->InitWalletFlags(wallet_creation_flags | WALLET_FLAG_LAST_HARDENED_XPUB_CACHED);
 
         // Only descriptor wallets can be created
         assert(walletInstance->IsWalletFlagSet(WALLET_FLAG_DESCRIPTORS));