fix: src/Ombi.Notifications/Ombi.Notifications.csproj to reduce vulnerabilities (#5167)

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-DOTNET-BOUNCYCASTLECRYPTOGRAPHY-7831268
- https://snyk.io/vuln/SNYK-DOTNET-BOUNCYCASTLECRYPTOGRAPHY-7831266

Co-authored-by: snyk-bot <snyk-bot@snyk.io>

[skip-ci]

src/Ombi.Notifications/Ombi.Notifications.csproj

@@ -12,7 +12,7 @@
 
   <ItemGroup>
     <PackageReference Include="Ensure.That" Version="10.1.0" />
-    <PackageReference Include="MailKit" Version="4.1.0" />
+    <PackageReference Include="MailKit" Version="4.6.0" />
   </ItemGroup>
 
   <ItemGroup>