Prevent possible F#4.0 compilation issue

It seems that these attributes can cause this compilation error in F#4.0:
error FS0927: The kind of the type specified by its attributes does not match the kind implied by its definition

Newer versions of F# (like 4.5 or even newer, like the one being used
by .NETCore to build the binary that is later published in nuget) allow
compiling this code but could generate exceptions at runtime if the
resulting binary is referenced when compiling with an old compiler, e.g.:

System.TypeLoadException : Could not load type of field 'GWallet.Backend.UtxoCoin.Lightning.SerializedChannel:MinSafeDepth@' (6) due to: Expected reference type but got type kind 17

src/DotNetLightning.Core/Utils/Errors.fs

@@ -101,7 +101,6 @@ module OnionError =
     let CHANNEL_DISABLED = UPDATE ||| 20us
 
     [<Flags>]
-    [<Struct>]
     type FailureCode = | FailureCode of uint16 with
 
         member this.Value = let (FailureCode v) = this in v

src/DotNetLightning.Core/Utils/LNMoney.fs

@@ -20,7 +20,6 @@ type LNMoneyUnit =
 /// Why not use the package directly? because it might cause circular dependency in the future.
 /// (i.e. We might want to support this package in BTCPayServer.Lightning)
 /// refs: https://github.com/btcpayserver/BTCPayServer.Lightning/blob/f65a883a63bf607176a3b7b0baa94527ac592f5e/src/BTCPayServer.Lightning.Common/LightMoney.cs
-[<Struct>]
 type LNMoney = | LNMoney of int64 with
 
     static member private BitcoinStyle =

src/DotNetLightning.Core/Utils/Primitives.fs

@@ -32,7 +32,6 @@ module Primitives =
             output
 
     /// Absolute block height
-    [<Struct>]
     type BlockHeight = | BlockHeight of uint32 with
         static member Zero = 0u |> BlockHeight
         static member One = 1u |> BlockHeight
@@ -58,7 +57,7 @@ module Primitives =
     /// 16bit relative block height used for `OP_CSV` locks,
     /// Since OP_CSV allow only block number of 0 ~ 65535, it is safe
     /// to restrict into the range smaller than BlockHeight
-    and  [<Struct>] BlockHeightOffset16 = | BlockHeightOffset16 of uint16 with
+    and BlockHeightOffset16 = | BlockHeightOffset16 of uint16 with
         member x.Value = let (BlockHeightOffset16 v) = x in v
 
         static member ofBlockHeightOffset32(bho32: BlockHeightOffset32) =
@@ -77,7 +76,7 @@ module Primitives =
     ///
     /// 32bit relative block height. For `OP_CSV` locks, BlockHeightOffset16
     /// should be used instead.
-    and  [<Struct>] BlockHeightOffset32 = | BlockHeightOffset32 of uint32 with
+    and BlockHeightOffset32 = | BlockHeightOffset32 of uint32 with
         member x.Value = let (BlockHeightOffset32 v) = x in v
 
         static member ofBlockHeightOffset16(bho16: BlockHeightOffset16) =
@@ -336,23 +335,20 @@ module Primitives =
     type BlockId = | BlockId of uint256 with
         member x.Value = let (BlockId v) = x in v
 
-    [<Struct>]
     type HTLCId = | HTLCId of uint64 with
         static member Zero = HTLCId(0UL)
         member x.Value = let (HTLCId v) = x in v
 
         static member (+) (a: HTLCId, b: uint64) = (a.Value + b) |> HTLCId
 
-    [<Struct>]
     type TxOutIndex = | TxOutIndex of uint16 with
         member x.Value = let (TxOutIndex v) = x in v
 
-    [<Struct>]
     type TxIndexInBlock = | TxIndexInBlock of uint32 with
         member x.Value = let (TxIndexInBlock v) = x in v
 
 
-    [<Struct;StructuredFormatDisplay("{AsString}")>]
+    [<StructuredFormatDisplay("{AsString}")>]
     type ShortChannelId = {
         BlockHeight: BlockHeight
         BlockIndex: TxIndexInBlock

src/DotNetLightning.Core/Utils/UInt48.fs

@@ -3,7 +3,6 @@ namespace DotNetLightning.Utils
 open System
 open DotNetLightning.Core.Utils.Extensions
 
-[<Struct>]
 type UInt48 = {
     UInt64: uint64
 } with