Merge #19455: rpc generate: print useful help and error message

f0aa8aeea5a183ea44a877255d12db7732f2e0a8 test: add rpc_generate functional test (Jon Atack)
92d94ffb8d07cc0d2665c901de5903a3a90d5fd0 rpc: print useful help and error message for generate (Jon Atack)
8d32d2011d3f4e1d9e587d6f80dfa4a3e9f9393d test: consider generate covered in _get_uncovered_rpc_commands() (Jon Atack)

Pull request description:

  This was a requested follow-up to #19133 and #17700 to alleviate confusion and head-scratching by people following tutorials that use `generate`. See https://github.com/bitcoin/bitcoin/pull/19455#issuecomment-668172916 below, https://github.com/bitcoin/bitcoin/pull/19133#issuecomment-636860943 and https://github.com/bitcoin/bitcoin/pull/17700#issuecomment-566159096.

  before
  ```
  $ bitcoin-cli help generate
  help: unknown command: generate

  $ bitcoin-cli generate
  error code: -32601
  error message:
  Method not found
  ```

  after
  ```
  $ bitcoin-cli help generate
  generate ( nblocks maxtries ) has been replaced by the -generate cli option. Refer to -help for more information.

  $ bitcoin-cli generate
  error code: -32601
  error message:
  generate ( nblocks maxtries ) has been replaced by the -generate cli option. Refer to -help for more information.
  ```

  In the general help it remains hidden, as requested by laanwj.
  ```
  $ bitcoin-cli help

  == Generating ==
  generateblock "output" ["rawtx/txid",...]
  generatetoaddress nblocks "address" ( maxtries )
  generatetodescriptor num_blocks "descriptor" ( maxtries )
  ```

ACKs for top commit:
  adamjonas:
    utACK f0aa8aeea5a183ea44a877255d12db7732f2e0a8
  pinheadmz:
    ACK f0aa8aeea5a183ea44a877255d12db7732f2e0a8

Tree-SHA512: d083652589ad3e8228c733455245001db22397559c3946e7e573cf9bd01c46e9e88b72d934728ec7f4361436ae4c74adb8f579670b09f479011924357e729af5

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