Make NRT enabled the default setting for projects and CI builds

[axunonb]

• Auto-replace #nullable enable per file with empty string
• Add nullable setting for all project files (Ical.Net: enable, Ical.Net.Tests: disable)
• Update GitHub Workflows nullability parameter: Now uses project settings
• Migrate Ical.Net.Benchmarks to NRT enabled

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[axunonb]

Couldn't yet find a way to get rid of the nullability warnings for netstandard2.0 on ubuntu. They don't appear on Windows.

[minichma]

Congrats, with all the previous NRT PRs this was a significant piece work. Thanks for the nice commit structure of this one, made the review a breeze.

[axunonb]

Thanks for the review. The NRT for the ical.net project is finally completed. Implementing NRT for the unit tests is less painful, and we can handle it one file at a time.