Added .NET 5 target runtime

[aivascu]

Description

• Raised the minimum .NET Framework version to 4.6.2
• Removed .NET Core 2 and .NET Core 3 target runtimes from test projects
• Added net48 target to test projects to test netstandard2.0 target
• Added .NET 5 target runtime to all projects
• Fixed failing tests
• Fixed analyzer warnings
• Raised minimum supported version of NUnit to 3.7.0
• Raised minimum supported version of xunit.extensibility.core to 2.2.0
• Added packages fixing transitive vulnerabilities

Closed issues

N/A

Checklist

• Reviewed the contribution guidelines
• Linked the issue(s) the PR closes
• Implemented automated tests and checked coverage
• Provided inline documentation comments for new public API
• Ran the full solution build and validation locally