Update target frameworks of test and benchmark projects

[martinnormark]

This pull request includes updates to the target framework and package references for the PreMailer.Net project. This will address the failing tests.

The most important changes are as follows:

Framework updates:

• Updated the target framework from net7.0 to net9.0 in PreMailer.Net/Benchmarks/Benchmarks.csproj.
• Updated the target framework from net7.0 to net9.0 in PreMailer.Net/PreMailer.Net.Tests/PreMailer.Net.Tests.csproj.

Package reference updates:

• Updated BenchmarkDotNet from version 0.13.3 to 0.14.0 in PreMailer.Net/Benchmarks/Benchmarks.csproj.
• Updated several package references in PreMailer.Net/PreMailer.Net.Tests/PreMailer.Net.Tests.csproj:
  • Microsoft.NET.Test.Sdk from version 17.9.0 to 17.12.0
  • Moq from version 4.20.70 to 4.20.72
  • xunit from version 2.7.0 to 2.9.3
  • xunit.runner.visualstudio from version 2.5.7 to 3.0.1
  • coverlet.collector from version 6.0.2 to 6.0.4