feature: Netstandard2.1

[jerge]

Summary

This PR adds support for netstandard2.1 as requested.

Changes

• Sets the target framework of the main library to netstandard2.1.
• Adds an explicit reference to System.Text.Json version 6.0.11 (compatible with netstandard2.1).
• Adds an explicit reference to System.Collections.Immutable version 9.0.4.
• Replaces UnconditionalSuppressMessage with SuppressMessage due to visibility constraints in netstandard2.1.
• Updates the test project to target net8.0 (required to support System.Collections.Immutable 9.0.4).