dep: Update ReactiveUI to v20 (major)

[renovate[bot]]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
ReactiveUI (source)	19.6.12 -> 20.0.1
ReactiveUI.WPF (source)	19.6.12 -> 20.0.1
ReactiveUI.WinUI (source)	19.6.12 -> 20.0.1

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Release Notes

reactiveui/reactiveui (ReactiveUI)

### [`v20.0.1`](https://togithub.com/reactiveui/ReactiveUI/releases/tag/20.0.1) [Compare Source](https://togithub.com/reactiveui/reactiveui/compare/19.6.12...20.0.1) ##### Housekeeping: - [`8e33bf4`](https://togithub.com/reactiveui/reactiveui/commit/8e33bf489894f3116914b8715653f8d63329255c) Housekeeping: Retire Xamarin due to end of support ([#​3797](https://togithub.com/reactiveui/reactiveui/issues/3797)) [@​ChrisPulman](https://togithub.com/ChrisPulman) - [`a31490e`](https://togithub.com/reactiveui/reactiveui/commit/a31490ef46469e4b96bf2b4201dd224263e2e514) Housekeeping Update Build to use dotnet build ([#​3799](https://togithub.com/reactiveui/reactiveui/issues/3799)) [@​ChrisPulman](https://togithub.com/ChrisPulman) - [`8b2f351`](https://togithub.com/reactiveui/reactiveui/commit/8b2f3519243facf961a1dd5feeb07dcba378345d) Housekeeping Tidy using's ([#​3798](https://togithub.com/reactiveui/reactiveui/issues/3798)) [@​ChrisPulman](https://togithub.com/ChrisPulman) - [`5ab874e`](https://togithub.com/reactiveui/reactiveui/commit/5ab874e572a6ad406b939ad5b46b92c603df4a57) Housekeeping: Prepare for Release ([#​3802](https://togithub.com/reactiveui/reactiveui/issues/3802)) [@​ChrisPulman](https://togithub.com/ChrisPulman)

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