Update MimeKit package because of a security issue

[axunonb]

PR Classification

This pull request focuses on code cleanup and modernization, specifically by enhancing test readability and consistency, adopting newer C# features, and updating dependencies.

PR Summary

The pull request modernizes the test suite and codebase of the MailMergeLib project by refactoring assertion syntax for better readability, adopting C# 8.0 nullable reference types, and updating NuGet package versions. Key changes include:

• Refactored assertion syntax across multiple test classes to use Assert.That with the NUnit constraint model.
• Introduced Assert.Multiple blocks in test methods for grouped assertion evaluation.
• Updated method signatures to accept nullable reference types, reflecting adoption of C# 8.0 features.
• Removed netcoreapp3.1 target framework and updated various NuGet package references in MailMergeLib.Tests.csproj.
• Added NUnit.Analyzers reference in MailMergeLib.Tests.csproj for enhanced test writing assistance.

Package Updates

MailMergeLib package updates

• MimeKit v4.6.0 => v4.7.1.1
• MailKit v4.6.0 => 4.7.1
• YAXLib v4.2.2 => v4.3.0

Unit test package updates

• Microsoft.NET.Test.Sdk 17.6.3 => 17.10.0
• NUnit v3.13.3 => v4.1.0
• Drop NetCore3.1
• Add Nunit.Analyzers v4.2.0