chainfee: change min conf target to be 1

This commit changes the minBlockTarget used by the WebAPIEstimator to be
1, inline with the bitcoind's accepted min conf target.

lntest/fee_service_test.go

@@ -34,6 +34,6 @@ func TestFeeService(t *testing.T) {
 	require.NoError(t, err)
 
 	require.Equal(
-		t, "{\"fee_by_block_target\":{\"2\":20000}}", string(body),
+		t, "{\"fee_by_block_target\":{\"1\":20000}}", string(body),
 	)
 }