Removed warnings, and added .NET 4 support

[artdolya]

No description provided.

[Copilot]

Pull request overview

This PR adds .NET Framework 4.x support and removes obsolete NLog configuration warnings by replacing the deprecated RequiredParameter attribute with manual validation logic in the InitializeTarget method.

• Adds support for .NET Framework 4.6.2, 4.7.2, and 4.8 target frameworks
• Replaces obsolete [RequiredParameter] attributes with explicit validation code
• Updates solution file to Visual Studio Version 18

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
src/Nlog.RabbitMQ.Target/RabbitMqTarget.cs	Adds manual validation for Fields configuration to replace deprecated RequiredParameter attributes, checking for null entries, empty keys, missing layouts, and duplicate keys
src/Nlog.RabbitMQ.Target/Nlog.RabbitMQ.Target.csproj	Adds net462, net472, and net48 target frameworks and extends LangVersion condition to include these new frameworks
src/Nlog.RabbitMQ.Target/Field.cs	Removes deprecated [RequiredParameter] attributes from Key and Layout properties
Nlog.RabbitMQ.sln	Updates Visual Studio version to 18 and adds Directory.Packages.props to solution items

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[Copilot]

The VisualStudioVersion value '18.3.11310.294 d18.3' appears malformed with an extra 'd18.3' suffix. Visual Studio version strings typically follow the format 'Major.Minor.Build.Revision' without additional suffixes. This could cause issues with tooling that parses this version string. The value should likely be '18.3.11310.294' without the 'd18.3' suffix.

Suggested change

	VisualStudioVersion = 18.3.11310.294 d18.3
	VisualStudioVersion = 18.3.11310.294

[snakefoot]

It doesn't make sense to add NET4, as the RabbitMQ.Client-nuget-package only supports NETStandard 2.0 + NET8.

Net462 only makes sense if you downgrade to RabbitMQ.Client ver. 6.8.1