Enable to run on .NET 8.0

[YusukeTobo]

Purpose

This sample application cannot run on .NET 8.0 (current LTS). This PR enable it to run .NET 8.

Does this introduce a breaking change?

[x ] Yes
[ ] No

Pull Request Type

upgraded TFM from .NET Core 3.1 to .NET 8.

[ x] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Documentation content changes
[ ] Other... Please describe:

How to Test

In Browser, I tested ToDo application operations.

• Test the code
  N/A

What to Check

Verify that the following are valid

• ...

Other Information

N/A