Upgrade to .NET 8 and do a new release

[keyuxuan]

Is your feature request related to a problem? Please describe.
We have packages that depends on McMaster.Extensions.CommandLineUtils, and the End of support for .NET 6 will be on November 12th, 2024, but in csproj files such as McMaster.Extensions.CommandLineUtils.csproj, its still target .NET 6.

Describe the solution you'd like
could you please upgrade to .NET 8 which is the LTS version and do a new release

Additional context

[github-actions]

This issue has been automatically marked as stale because it has no recent activity. It will be closed if no further activity occurs. Please comment if you believe this should remain open, otherwise it will be closed in 14 days. Thank you for your contributions to this project.

[github-actions]

Closing due to inactivity.
If you are looking at this issue in the future and think it should be reopened, please make a commented here and mention natemcmaster so he sees the notification.

[natemcmaster]

@Chris-Wolfgang sent a PR to upgrade. I'll work on getting a release out.