Add net6.0 target framework for Bugsnag and Bugsnag.AspNet.Core

[yousif-bugsnag]

Goal

Adds a net6.0 target framework for the Bugsnag and Bugsnag.AspNet.Core packages and updates project references accordingly.

Design

Many of the package references (nuget dependencies) for these projects are provided by the .NET platform/framework and only require a nuget package reference when targeting netstandard.

By targeting net6.0 and restricting these package references to the netstandard2.0 target, consuming projects on .NET 6+ will not need to pull in unnecessary nuget dependencies for packages that are provided by the platform/runtime, while projects that still need to use the netstandard2.0 dll can continue to do so.

In addition, I've removed the DisableImplicitFrameworkReferences config and removed unnecessary framework references from the project files. This was an outdated approach from the early days of netstandard that is no longer required/recommended for netstandard2.0 (see https://github.com/dotnet/standard/blob/release/2.0.0/docs/faq.md#should-i-reference-the-meta-package-or-should-i-reference-individual-packages).

Testing

Tested manually

[twometresteve]

My only comment is should we have a CHANGELOG entry for this?

[yousif-bugsnag]

I'm planning to do the changelog separately as part of the release PR