fix: lower ChannelCloseMinimum fee floor to prevent cooperative close failures

Bypass the FEERATE_FLOOR_SATS_PER_KW (253 sat/kwu) enforcement for
ChannelCloseMinimum in both ldk-node's OnchainFeeEstimator and
rust-lightning's calculate_closing_fee_limits. This prevents cooperative
close negotiation from failing when the counterparty (e.g. LND LSP)
proposes fees below the standard 1 sat/vbyte relay floor, which is
common on signet/testnet and can occur on mainnet during low-fee periods.

Without this fix, every cooperative close attempt falls back to a force
close due to "Unable to come to consensus about closing feerate".

Author: kaloudis