Proposal:Please remove WinUI 3 UWP Mode limitation

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# Proposal: Please remove WinUI 3 UWP Mode limitation
Hope to remove the restriction of UWP Mode (CoreApplication) in Winui 3 1.5
希望能在Winui 3 1.5中移除 UWP 模式 (CoreApplication)的限制

## Summary
At present, in WinUI 3 1.5 Experimental 1, we found that some methods have added support for UWP (CoreApplication) Mode, but the program to create WinUI 3 UWP Mode(CoreApplication), However, it was found that internal anomalies still occurred during startup, which caused the application to fail to start normally. I hope to remove this restriction as soon as possible, and bring back the missing UWP template in WinUI 3 1.0 Experimental 1. And upgrade the TargetFrameWork version of the C# UWP template to.Net Core 8.0

目前在WinUI 3 1.5 Experimental 1 中，我们发现了一部分方法中已经添加了对 UWP (CoreApplication) 模式的支持，但创建 WinUI 3 UWP Mode(CoreApplication) 的程序，在启动时却发现仍然会发生内部异常导致应用无法正常启动，希望能尽快移除这一限制，并带回 WinUI 3 1.0 Experimental 1 中消失的 UWP 模板，并升级 C# UWP 模板的 TargetFrameWork 版本到 .Net Core 8.0

IApplicationOverridesFeature_UwpSupportApi interface

## Rationale

See below

## Scope

| Capability | Priority |
| :---------- | :------- |
| This proposal will allow developers to accomplish W | Must |
| This proposal will allow end users to accomplish X | Should |
| This proposal will allow developers to accomplish Y | Could |
| This proposal will allow end users to accomplish Z | Won't |

## Important Notes

## Open Questions

Start with the linked WinUI feature proposal process and the IApplicationOverridesFeature_UwpSupportApi interface named in the issue. Investigate the reported startup exception in WinUI 3 UWP/CoreApplication mode and the missing UWP template. Done would require a defined scope and confirmation that the application starts normally, with the requested template and target framework changes addressed.

Written by the indexing model from the issue text.