Add .NET 6 target, enable NRT.

[tillig]

• Add .NET 6 target (both Autofac and Castle.Core target .NET 6).
• Enable nullable reference types.
• Update build metadata (analyzer rules, .editorconfig).
• Resolve analyzer warnings. The big changes for this were in the tests - names being too terse (IHasI for an interface, C for class names) and conflicts between property and method names (property named I, method named GetI).

The semver for this is already 7.0.0 for the previous PR that updated the Castle.Core version, so after this goes in I think we're good to release.

[codecov[bot]]

Codecov Report

Patch coverage: 87.50% and project coverage change: +12.64 :tada:

Comparison is base (92b54a7) 80.80% compared to head (44b8e1b) 93.44%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #54 +/- ## ============================================ + Coverage 80.80% 93.44% +12.64% ============================================ Files 1 1 Lines 125 122 -3 Branches 24 22 -2 ============================================ + Hits 101 114 +13 + Misses 14 4 -10 + Partials 10 4 -6 ``` | [Impacted Files](https://app.codecov.io/gh/autofac/Autofac.Extras.DynamicProxy/pull/54?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=autofac) | Coverage Δ | | |---|---|---| | [...ofac.Extras.DynamicProxy/RegistrationExtensions.cs](https://app.codecov.io/gh/autofac/Autofac.Extras.DynamicProxy/pull/54?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=autofac#diff-c3JjL0F1dG9mYWMuRXh0cmFzLkR5bmFtaWNQcm94eS9SZWdpc3RyYXRpb25FeHRlbnNpb25zLmNz) | `93.44% <87.50%> (+12.64%)` | :arrow_up: |

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[tillig]

It... appears I didn't add net6 to the TFM list like I thought I did. Not sure how that slipped past. I'll add that in so it shows up in the next release.