Update to .NET 10

[abart27]

Rationale is that this will allow us to use the Application.SetColorMode API to add back dark mode.

That API exists since .NET 9, but 10 is an LTS release so might as well

[Copilot]

Pull request overview

This PR updates the WinForms application project to target .NET 10, aligning with the intent to enable newer WinForms APIs such as Application.SetColorMode.

Changes:

• Updates the main STROOP application target framework from .NET 8 Windows to .NET 10 Windows.

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[FramePerfection]

Meant to put this as the summary comment of the review just now (oops):
I've updated the readme because I don't like lying to people 😉

Also sorry for letting this PR rot for so long, I didn't feel much immediate pressure to merge this because #16 entails quite a few things beyond just the WinForms theme API or whatever, since there are a couple of custom controls that have hardcoded color values themselves (most notably the variable panels) that would need to be adjusted.
I've also considered how .NET 10 is not yet a suitable candidate for cross-platform support, as the Linux distros that I am familiar with lag behind quite a bit (the most popular ones are still at best at .NET 8 as far as I can tell), but given that Linux support is definitely a whole different can of worms, I've come to accept that that vague thought shouldn't be (even a mental) blocker for this.

So... yeah, looks good to me now 😪👍

[abart27]

: because #16 entails quite a few things beyond just the WinForms theme API or whatever, since there are a couple of custom controls that have hardcoded color values themselves

I've started work on that already with no PR yet

https://github.com/FramePerfection/STROOP/commits/dark-mode/