lnwallet: increase legacy fee limit threshold to 1k sats

In this commit, we increase the legacy fee limit threshold (the amount
below which we'll allow 100% of funds to go to fees for the non-v2 RPC
calls) from 50 sats to 1k sats.

docs/release-notes/release-notes-0.14.2.md

@@ -22,13 +22,14 @@ connection from the watch-only node.
 * A bug that allowed fees to be up to 100% of the payment amount was fixed by
   [introducing a more sane default
   value](https://github.com/lightningnetwork/lnd/pull/6226) of 5% routing fees
-  (except for small amounts <= 50 satoshis where the 100% routing fees are kept
-  to accommodate for the base fee in channels). To avoid falling back to a
-  default value, users should always set their own fee limits by using the
-  `--fee_limit` or `--fee_limit_percent` flags on the `lncli payinvoice`,
-  `lncli sendpayment` and `lncli queryroutes` commands. Users of the gRPC or
-  REST API should set the `fee_limit` field on the corresponding calls
-  (`SendPayment`, `SendPaymentSync`, `QueryRoutes`).
+  (except for small amounts <= [1k
+  satoshis](https://github.com/lightningnetwork/lnd/pull/6234) where the 100%
+  routing fees are kept to accommodate for the base fee in channels). To avoid
+  falling back to a default value, users should always set their own fee limits
+  by using the `--fee_limit` or `--fee_limit_percent` flags on the `lncli
+  payinvoice`, `lncli sendpayment` and `lncli queryroutes` commands. Users of
+  the gRPC or REST API should set the `fee_limit` field on the corresponding
+  calls (`SendPayment`, `SendPaymentSync`, `QueryRoutes`).
 
 ## Database