Bump Mailjet.Api from 3.0.1 to 4.0.0 (#872)

Updated [Mailjet.Api](https://github.com/mailjet/mailjet-apiv3-dotnet)
from 3.0.1 to 4.0.0.

<details>
<summary>Release notes</summary>

_Sourced from [Mailjet.Api's
releases](https://github.com/mailjet/mailjet-apiv3-dotnet/releases)._

## 4.0.0



Commits viewable in [compare
view](mailjet/mailjet-apiv3-dotnet@v3.0.1...v4.0.0).
</details>

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Directory.Packages.props

@@ -25,7 +25,7 @@
     <PackageVersion Include="EssentialCSharp.Shared.Models" Version="$(ToolingPackagesVersion)" />
     <PackageVersion Include="HtmlAgilityPack" Version="1.12.4" />
     <PackageVersion Include="IntelliTect.Multitool" Version="1.5.3" />
-    <PackageVersion Include="Mailjet.Api" Version="3.0.1" />
+    <PackageVersion Include="Mailjet.Api" Version="4.0.0" />
     <PackageVersion Include="Microsoft.AspNetCore.Authentication.MicrosoftAccount" Version="10.0.3" />
     <PackageVersion Include="Microsoft.AspNetCore.Identity.EntityFrameworkCore" Version="10.0.3" />
     <PackageVersion Include="Microsoft.AspNetCore.Identity.UI" Version="10.0.3" />