Update Background tasks article for 3.0 (Generic Host)

[tdykstra]

Migrate sample app to 3.0 (Generic Host) and put snippets in 2.2 vs. 3.0 moniker blocks

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Document Details

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• ID: c3460b0d-9c60-3a06-ac63-18cf990e53c3
• Version Independent ID: b2dece1f-15b8-03af-6d52-7cc070432b4b
• Content: Background tasks with hosted services in ASP.NET Core
• Content Source: aspnetcore/fundamentals/host/hosted-services.md
• Product: aspnet-core
• Technology: aspnetcore-fundamentals
• GitHub Login: @guardrex
• Microsoft Alias: riande

[guardrex]

Confirm and enable worker service content for VS4Mac ...

# [Visual Studio for Mac](#tab/visual-studio-mac)

1. Create a new project with **File** > **New Solution**.
1. Select **App** under **.NET Core** in the sidebar.
1. Select **Worker** under **ASP.NET Core**. Select **Next**.
1. Select **.NET Core 3.0** for the **Target Framework**. Select **Next**.
1. Provide a name in the **Project Name** field. Select **Create**.

[guardrex]

@davidfowl @anurse The hosted service topic has separate 2.x samples: Web Host and Generic Host.

❓ Would you like the 3.x updates to include a 3.x Web Host sample, or should I drop that sample and only have a 3.x Generic Host sample going forward?

... and a follow-up is that since the Generic Host is a bit different from 2.x days, should we even keep that example in the 2.x version of the topic? The 2.x version really only relies on the Web Host sample app for its code and explanations.