Enable nullable feature on GitVersion.Core

[Evangelink]

Description

Enable the nullable feature on GitVersion.Core project.

Related Issue

Fixes #2764

[Evangelink]

I will rebase this PR once the readonly and the analyzers got merged.

[asbjornu]

Great stuff!

[asbjornu]

This is great! I love that it's now visible how many opportunities for NullReferenceException we have in the codebase. 😅 That's something I would very much like to remedy, though. I'm not saying all ? can or should be removed, but I do think there's way too many in the current codebase.

If you see any places where null guards would alleviate the need for null values, please sprinkle them throughout the codebase as you see fit. As long as the tests are green; the more null guards and less ? we have, the better.

That's also something we can do in a followup PR. If you'd like this merged now, that's fine by me.

[mergify]

Thank you @Evangelink for your contribution!

[arturcic]

@Evangelink thank you for the effort. I know what this kind of refactoring is so many thanks. Hope we can improve the codebase even more so that we can prepare it for version 6.0 (and target net 6.0 as it's LTS and drop support for net48 and netcore3.1)