Remove Annotation nuspec file

[philasmar]

Description of changes:

• Remove Annotation's library nuspec file and replace with a regular dotnet build of the Amazon.Lambda.Annotations.SourceGenerator project to produce the Amazon.Lambda.Annotations package.
• Update tests to automatically replace the version of the Amazon.Lambda.Annotations.SourceGenerator project instead of needing to manually update them every time there is a version bump.
• Add the .NET8 target with trimming to the NuGet package as it was not previously added
• Removed dev preview text from the package description

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.