bump: [DotNet] Update generators and skill bots to NET 8

[JhontSouth]

Fixes #1559 #minor

Purpose

This PR changes the .NET version used from .NET 6.0 to .NET 8.0 in the bot generators and the SDK samples.

Changes

• Update defaultSdkVersion from 4.21.2 to 4.22.4.
• Update to .NET 8.0 the base generator template.
• Update to .NET 8.0 for all skill projects and test projects.
• Update to .NET 8.0 the functional tests projects.
• Fix CalendarSkill references.

Tests

The following image shows a bot generated with the .NET 8 version and version 4.22.4 for the BotBuilder SDK packages.