Merge bitcoin/bitcoin#30275: Fee Estimation: change `estimatesmartfee` default mode to `economical`

25bf86a225b0df3f48ade1016b47f5ee1636b988 [test]: ensure `estimatesmartfee` default mode is `economical` (ismaelsadeeq)
41a2545046bce315af697a3c6baf6e3fb2e824c2 [fees]: change `estimatesmartfee` default mode to `economical` (ismaelsadeeq)

Pull request description:

  Fixes #30009

  This PR changes the `estimatesmartfee` default mode to `economical`.

  This was also suggested on IRC https://bitcoin-irc.chaincode.com/bitcoin-core-dev/2024-04-26#1021609

  - `conservative` mode: This is the `estimatesmartfee` RPC mode which considers a longer history of blocks. It potentially returns a higher fee rate and is more likely to be sufficient for the desired target, but it is not as responsive to short-term drops in the prevailing fee market.
  - `economical` mode: This is the `estimatesmartfee` RPC mode where estimates are potentially lower and more responsive to short-term drops in the prevailing fee market.

  Since users are likely to use the default mode, this change will reduce overestimation for many users. The conservative mode remains available for those who wish to opt-in.

  For an in-depth analysis of how significantly the `conservative` mode overestimates, see
  https://delvingbitcoin.org/t/bitcoind-policy-estimator-modes-analysis/964.

ACKs for top commit:
  instagibbs:
    reACK https://github.com/bitcoin/bitcoin/pull/30275/commits/25bf86a225b0df3f48ade1016b47f5ee1636b988
  glozow:
    ACK 25bf86a225b0df3f48ade1016b47f5ee1636b988
  willcl-ark:
    ACK 25bf86a225b0df3f48ade1016b47f5ee1636b988

Tree-SHA512: 78ebda667eb9c8f87dcc2f0e6c14968bd1de30358dc77a13611b186fb8427ad97d9f537bad6e32e0a1aa477ccd8c64fee4d41e19308ef3cb184ff1664e6ba8a6

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