Merge #20403: wallet: upgradewallet fixes, improvements, test coverage

3eb6f8b2e61c24a22ea9396d86672307845f35eb wallet (not for backport): improve upgradewallet error messages (Jon Atack)
ca8cd893bb56bf5d455154b0498b1f58f77d20ed wallet: fix and improve upgradewallet error responses (Jon Atack)
99d56e357159c7154f69f28cb5587c5ca20d6594 wallet: fix and improve upgradewallet result responses (Jon Atack)
2498b04ce88696a3216fc38b7d393906b733e8b1 Don't upgrade to HD split if it is already supported (Andrew Chow)
c46c18b788cb0862aafbb116fd37936cbed6a431 wallet: refactor GetClosestWalletFeature() (Jon Atack)

Pull request description:

  This follows up on #18836 and #20282 to fix and improve the as-yet unreleased `upgradewallet` feature and also implement review follow-up in https://github.com/bitcoin/bitcoin/pull/18836#discussion_r519328607.

  This PR fixes 4 upgradewallet issues:

  - this bug: https://github.com/bitcoin/bitcoin/pull/20403#discussion_r526063920
  - it returns nothing in the absence of an RPC error, which isn't reassuring for users
  - it returns the same thing both in the case of a successful upgrade and when no upgrade took place
  - the error message object is currently dead code

  This PR fixes the above and provides:

  ...user feedback to not silently return without upgrading
  ```
  {
    "wallet_name": "disable private keys",
    "previous_version": 169900,
    "current_version": 169900,
    "result": "Already at latest version. Wallet version unchanged."
  }
  ```
  ...better feedback after successfully upgrading
  ```
  {
    "wallet_name": "watch-only",
    "previous_version": 159900,
    "current_version": 169900,
    "result": "Wallet upgraded successfully from version 159900 to version 169900."
  }
  ```
  ...helpful error responses
  ```
  {
    "wallet_name": "blank",
    "previous_version": 169900,
    "current_version": 169900,
    "error": "Cannot downgrade wallet from version 169900 to version 159900. Wallet version unchanged."
  }
  {
    "wallet_name": "blank",
    "previous_version": 130000,
    "current_version": 130000,
    "error": "Cannot upgrade a non HD split wallet from version 130000 to version 169899 without upgrading to support pre-split keypool. Please use version 169900 or no version specified."
  }
  ```
  updated help:
  ```
  upgradewallet ( version )

  Upgrade the wallet. Upgrades to the latest version if no version number is specified.
  New keys may be generated and a new wallet backup will need to be made.
  Arguments:
  1. version    (numeric, optional, default=169900) The version number to upgrade to. Default is the latest wallet version.

  Result:
  {                            (json object)
    "wallet_name" : "str",     (string) Name of wallet this operation was performed on
    "previous_version" : n,    (numeric) Version of wallet before this operation
    "current_version" : n,     (numeric) Version of wallet after this operation
    "result" : "str",          (string, optional) Description of result, if no error
    "error" : "str"            (string, optional) Error message (if there is one)
  }
  ```

ACKs for top commit:
  achow101:
    ACK  3eb6f8b
  MarcoFalke:
    review ACK 3eb6f8b2e61c24a22ea9396d86672307845f35eb 🛡

Tree-SHA512: b767314069e26b5933b123acfea6aa40708507f504bdb22884da020a4ca1332af38a7072b061e36281533af9f4e236d94d3c129daf6fe5b55241127537038eed

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