Update Uno Platform library TFMs to net8.0

[Arlodotexe]

Background

Our Uno Platform TFMs are currently set to net7.0*, originally upgraded in https://github.com/CommunityToolkit/Tooling-Windows-Submodule/pull/117.

The problem

However, .NET 7 will reach End of Support on May 14, 2024.

The solution

Since .NET 8 is an LTS release and will be supported until November 10, 2026, we should aim to onboard our users to this version before .NET 7 hits end of support, and (far) before .NET 9 is released.

People can take a long time to update their projects, so we should prefer targeting LTS instead of STS when LTS is newer. The sooner we onboard consumers to the newest LTS, the healthier the ecosystem in the long term.

Additional info

See the official .NET and .NET Core Support Policy for details and guidance.