Bump MailKit from 2.13.0 to 2.15.0 (#9)

Bumps [MailKit](https://github.com/jstedfast/MailKit) from 2.13.0 to 2.15.0.
<details>
<summary>Changelog</summary>

*Sourced from [MailKit's changelog](https://github.com/jstedfast/MailKit/blob/master/ReleaseNotes.md).*

> ### MailKit 2.15.0 (2021-08-18)
>
> * Use DebugType=full for .NET Framework v4.x. (issue [#1239](jstedfast/MailKit#1239))
> * Updated GMail SSL certificate serial numbers and fingerprints.
> * Small NTLM code improvements.
>
> ### MailKit 2.14.0 (2021-07-28)
>
> * Added support for logging timestamps in the `ProtocolLogger` (see the `LogTimestamps` and `TimestampFormat`
>   properties on `ProtocolLogger`).
> * Added support for automatically redacting user credentials in protocol logs. To enable this, set the
>   `ProtocolLogger.RedactSecrets` property to `true`. (issue [#1174](jstedfast/MailKit#1174))
> * Added the GetMessageSizeAsync() method to the IMailSpool interface.
>   (issue [#1233](jstedfast/MailKit#1233))
> * Added a work-around to the IMAP INTERNALDATE parser to handle invalid dates such as "00-Jan-0000 00:00:00 +0000"
>   which appears in Domino IMAP server responses, likely when the INTERNALDATE value is uninitialized in the database.
>   (issue [#1236](jstedfast/MailKit#1236))
> * Make sure to dispose X509Certificates in .NET >= 4.6.
> * Re-added NTLM as one of the default supported SASL mechanisms.
> * Updated GMail SSL certificate serial numbers and fingerprints.
</details>
<details>
<summary>Commits</summary>

- [`6ff7091`](jstedfast/MailKit@6ff7091) Bumped version to 2.15.0
- [`a645991`](jstedfast/MailKit@a645991) Bump Microsoft.NET.Test.Sdk from 16.10.0 to 16.11.0 ([#1242](jstedfast/MailKit#1242))
- [`caa02b9`](jstedfast/MailKit@caa02b9) NTLM improvements
- [`b206436`](jstedfast/MailKit@b206436) Drop expired GMail SSL certificates
- [`945508f`](jstedfast/MailKit@945508f) Updated GMail SSL certificate serial #'s and fingerprints
- [`e188a02`](jstedfast/MailKit@e188a02) Bump NUnit3TestAdapter from 3.17.0 to 4.0.0 ([#1218](jstedfast/MailKit#1218))
- [`3f54627`](jstedfast/MailKit@3f54627) Use DebugType=full for .NET Framework v4.x
- [`1e34019`](jstedfast/MailKit@1e34019) Bumped version to 2.14.0
- [`ca0751c`](jstedfast/MailKit@ca0751c) If an invalid INTERNALDATE is encountered, just use DateTimeOffset.MinValue
- [`f1323e6`](jstedfast/MailKit@f1323e6) Update GMail SSL certificate info
- Additional commits viewable in [compare view](jstedfast/MailKit@2.13.0...2.15.0)
</details>

<br />

Reviewed-on: https://gitea.dysnomia.studio/elanis/dysnomia-website/pulls/9
Co-authored-by: elanis <elanis@noreply.example.org>
Co-committed-by: elanis <elanis@noreply.example.org>

Dysnomia.Website.WebApp/Dysnomia.Website.WebApp.csproj

@@ -1,4 +1,4 @@
-<Project Sdk="Microsoft.NET.Sdk.Web">
+<Project Sdk="Microsoft.NET.Sdk.Web">
 
   <PropertyGroup>
     <TargetFramework>net5.0</TargetFramework>
@@ -7,7 +7,7 @@
 
   <ItemGroup>
     <PackageReference Include="Dysnomia.Common.Stats" Version="1.0.4" />
-    <PackageReference Include="MailKit" Version="2.13.0" />
+    <PackageReference Include="MailKit" Version="2.15.0" />
     <PackageReference Include="reCAPTCHA.AspNetCore" Version="3.0.10" />
   </ItemGroup>