Bring `getmininginfo` up to date

Update the the testnet boolean attribute to chain string for network name.

On lbcd:

lbcctl getmininginfo
{
  "blocks": 1176991,
  "currentblocksize": 99937,
  "currentblockweight": 399640,
  "currentblocktx": 124,
  "difficulty": 1873348963350.1497,
  "errors": "",
  "generate": false,
  "genproclimit": 16,
  "hashespersec": 0,
  "networkhashps": 543153148851407,
  "pooledtx": 212,
  "testnet": false
}

Latest spec:

{                              (json object)
  "blocks" : n,                (numeric) The current block
  "currentblockweight" : n,    (numeric, optional) The block weight of the last assembled block (only present if a block was ever assembled)
  "currentblocktx" : n,        (numeric, optional) The number of block transactions of the last assembled block (only present if a block was ever assembled)
  "difficulty" : n,            (numeric) The current difficulty
  "networkhashps" : n,         (numeric) The network hashes per second
  "pooledtx" : n,              (numeric) The size of the mempool
  "chain" : "str",             (string) current network name (main, test, signet, regtest)
  "warnings" : "str"           (string) any network and blockchain warnings
}