Frontend.Console: target .NET8.0 instead of .net6

This is to fix this compiler error when building in Release
mode:

```
Determining projects to restore...
/usr/share/dotnet/sdk/10.0.100/Sdks/Microsoft.NET.Sdk/targets/Microsoft.NET.EolTargetFrameworks.targets(32,5): error NETSDK1138: The target framework 'net6.0' is out of support and will not receive security updates in the future. Please refer to https://aka.ms/dotnet-core-support for more information about the support policy. [/home/runner/work/geewallet/geewallet/src/GWallet.Frontend.Console/GWallet.Frontend.Console.fsproj]
```

Author: knocte

src/GWallet.Frontend.Console/GWallet.Frontend.Console.fsproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
     <OutputType>Exe</OutputType>
-    <TargetFramework>net6.0</TargetFramework>
+    <TargetFramework>net8.0</TargetFramework>
   </PropertyGroup>
   <ItemGroup>
     <Compile Include="..\GWallet.Backend\Properties\CommonAssemblyInfo.fs">