Disable nullable explicitly

[Pliner]

Hey,

We use LightInject(a source code package) as a built-in container for EasyNetQ.

We started a migration to Nullable Reference Types recently and observed build warnings because LightInject doesn't support them(and this is fine).

Could you consider adding #nullable disable pragma? It looks like the simplest way how these warnings could be suppressed.

Anyway, thanks for LightInject 🚀

[Pliner]

This PR fixes #552

[Pliner]

@seesharper Could you review this PR? 🙏🏻

[seesharper]

Thanks. Makes sense.

[Pliner]

@seesharper Thanks.

/home/runner/work/LightInject/LightInject/src/LightInject/LightInject.cs(1,2): 
error CS8370: Feature 'nullable reference types' is not available in C# 7.3. 
Please use language version 8.0 or greater. 
[/home/runner/work/LightInject/LightInject/src/LightInject/LightInject.csproj]

Let me check what could we do with this error 😞

[Pliner]

There are the following target frameworks:

<TargetFrameworks>net6.0;netstandard2.0;netcoreapp3.1</TargetFrameworks>

Okay, it looks like we have at least these two options:

1. Just bump a language version to 8.0 by adding <LangVersion>8.0</LangVersion>. 7.3 looks pretty old, doesn't it? :) This change will affect only netstandard2 source package users who still uses the default version language(7.3).
2. To introduce a constant, for example, USE_NULLABLE_DISABLE, and initialise it with true for netcoreapp3.1 and net6.0. Source package users will also be able to initialise the constant depending on their curcumstances.

@seesharper Which approach do you prefer?

PS I've committed a fix using the first approach just to speed up merging if you are happy with it.

[seesharper]

Thanks

[Pliner]

@seesharper Sorry for bothering you again about this small change. There is a version bump to 6.6.0, but no matching release exists. Could you release it please?

[Pliner]

@seesharper Thanks for the release! I've checked and it works perfectly 👍🏻