As written by others, .NET Framework is tied to Windows.
Microsoft used to offer 10 years of support on each previous Windows version (e.g. Windows 10 from 2015 to 2025)
As .NET 4.8 is integrated in Windows 11, we can expect support until 2031
vs 2024 for .NET 6.
But Microsoft is also saying that .NET 4.8 will only receive security fixes, and no new feature.
So the real question is how to you plan to support YOUR WPF App?
If nothing has to to change and stay as it is untouched on Windows machines, stick with .NET 4.8
If you plan to add more features in your app, and want to keep you skills up to date, moving to .NET 6 (and latter 8 and 10) would be a good option. You then need maintenance work once every ~2 years to update to next LTS version.
You can also take a mid path: first move to 4.8 (easy one) and then wait 2+ more years to migrate to .NET 8 once the MAUI is more established and then maybe consider to upgrade from WPF to MAUI.
Moving to .NET 8 with MAUI will for instance allow the deployment of your app on Linux or MacOS if at some point it become interesting for you.
.NET 8 shall be the next LTS, from end 2023 to end 2026 (so still shorter support than 4.8).
If Microsoft keeps the same release rhythm, and drop .NET 4.8 in next Windows 12 (a lot of if), we would have to wait for .NET 14 to get longer support than 4.8!
(6 2021-24, 8 2023-26, 10 2025-28, 12 2027-30, 14 2029-2032)
The option with less effort is definitely to chose 4.8 !