Upgrade all .NET 6 projects to .NET 8

# Description

The AWS Serverless Patterns repository currently contains examples using .NET 6, which is no longer the latest Long Term Support (LTS) version supported by AWS Lambda. To ensure we're demonstrating best practices and providing up-to-date code samples, we should upgrade all .NET 6 projects to .NET 8.

## Objectives

1. Identify all .NET 6 projects within the repository
2. Upgrade each project to .NET 8
3. Update any dependencies to versions compatible with .NET 8
4. Test each upgraded project to ensure functionality is maintained
5. Update documentation to reflect the use of .NET 8

## Benefits

- Demonstrates use of the latest LTS version of .NET supported by AWS Lambda
- Provides users with current best practices for serverless .NET development
- Ensures examples leverage the latest performance improvements and features in .NET 8

This upgrade will help maintain the repository's relevance and usefulness for developers working with AWS Lambda and .NET.