lower fsharp.core dependency to 5.x so users can float

This results in a nuspec that looks like this:

<?xml version="1.0" encoding="utf-8"?>
<package xmlns="http://schemas.microsoft.com/packaging/2013/05/nuspec.xsd">
  <metadata>
    <id>Saturn</id>
    <version>1.0.0</version>
    <authors>Krzysztof Cieslak</authors>
    <requireLicenseAcceptance>false</requireLicenseAcceptance>
    <license type="expression">MIT</license>
    <licenseUrl>https://licenses.nuget.org/MIT</licenseUrl>
    <projectUrl>https://saturnframework.org/</projectUrl>
    <iconUrl>https://avatars0.githubusercontent.com/u/35305523?s=200</iconUrl>
    <description>Opinionated, web development framework for F# which implements the server-side, functional MVC pattern</description>
    <copyright>Copyright 2020 Lambda Factory</copyright>
    <tags>F# web framework "ASP.Net Core" HTTP Service MVC functional</tags>
    <repository type="git" url="https://github.com/SaturnFramework/Saturn.git" />
    <dependencies>
      <group targetFramework="net5.0">
        <dependency id="FSharp.Control.Websockets" version="0.2.2" exclude="Build,Analyzers" />
        <dependency id="FSharp.Core" version="5.0.0" exclude="Build,Analyzers" />
        <dependency id="Giraffe" version="5.0.0" exclude="Build,Analyzers" />
        <dependency id="Microsoft.AspNetCore.Authentication.JwtBearer" version="5.0.7" exclude="Build,Analyzers" />
      </group>
    </dependencies>
    <frameworkReferences>
      <group targetFramework="net5.0">
        <frameworkReference name="Microsoft.AspNetCore.App" />
      </group>
    </frameworkReferences>
  </metadata>
</package>

SaturnFramework / Saturn

lower fsharp.core dependency to 5.x so users can float #310