ln update_fee: enforce that feerate is over default min relay fee

(this was always already the case when we are the funder, but we were
not checking it when remote is responsible for update_fee)

electrum/simple_config.py

@@ -34,6 +34,12 @@ FEERATE_STATIC_VALUES = [1000, 2000, 5000, 10000, 20000, 30000,
                          50000, 70000, 100000, 150000, 200000, 300000]
 FEERATE_REGTEST_HARDCODED = 180000  # for eclair compat
 
+# The min feerate_per_kw that can be used in lightning so that
+# the resulting onchain tx pays the min relay fee.
+# This would be FEERATE_DEFAULT_RELAY / 4 if not for rounding errors,
+# see https://github.com/ElementsProject/lightning/commit/2e687b9b352c9092b5e8bd4a688916ac50b44af0
+FEERATE_PER_KW_MIN_RELAY_LIGHTNING = 253
+
 FEE_RATIO_HIGH_WARNING = 0.05  # warn user if fee/amount for on-chain tx is higher than this