wallet, rpc: deprecate settxfee

Summary

This PR deprecates the settxfee RPC, marking it for removal in Bitcoin Core 30.0.

Motivation

The PR was initially motivated by https://github.com/bitcoin/bitcoin/issues/31088. The intention was to create a new function settxfeerate to allow users to set a static fee rate in sat/vB instead of btc/kvB.

The settxfee RPC allows users to set a static fee rate for all transactions created by the wallet. However, in a dynamic fee environment, this can lead to poor fee choices, either overpaying when the mempool is empty or underpaying when congestion is high. The preferred approach is to rely on fee estimation, which is designed to adapt to network conditions, and is the one by default.

During discussion the consensus was that static fee settings are a footgun and that users should instead specify the fee rate per transaction if they don't want to rely on the fee estimation. Given this, rather than introducing a settxfeerate alternative, this PR goes towards removing settxfee entirely.

Key Changes

settxfee is now deprecated and will be removed in Bitcoin Core 30.0. Users should rely on fee estimation or explicitly specify a fee rate when constructing transactions.

Impact on Users

If users currently use settxfee, they should transition to specifying fees per transaction. No immediate breakage in 29.0, but settxfee will be removed in 30.0.

Alternative Approaches Considered

A settxfeerate alternative (using sat/vB) was initially proposed but ultimately rejected in favor of deprecating static fee setting entirely.