build(deps): bump ReactiveUI.SourceGenerators to 4.2.0 (#4524)

## Summary

**ReactiveUI.Core now brings ReactiveUI.SourceGenerators 4.2.0.**

- **The generated WinForms hosts observe their properties through
ReactiveUI.Binding's `ObservedProperty`.** With Binding 8.4.0 or later,
which ReactiveUI brings once it moves to 8.4.0, `[RoutedControlHost]`
and `[ViewModelControlHost]` follow their properties with WhenAny
semantics instead of their own `PropertyChanged` handling. With older
Binding they keep their own handling, and RXUISG0022 (Info) names the
Binding version they need.

## Why

**Generated code cannot use `WhenAny` directly, because
ReactiveUI.Binding's generator never sees another generator's output.**
Binding 8.4.0 added `ObservedProperty` for this case, and
SourceGenerators 4.2.0 uses it.

## Breaking changes

**None.**

## How this was verified

**ReactiveUI.Core builds with warnings as errors, and the ReactiveUI
tests, including the source generator integration tests, pass against
4.2.0.**

## Notes for the reviewer

**One line in `src/Directory.Packages.props`.** The ReactiveUI.Binding
8.4.0 bump is a separate PR.

## Checklist
- [x] I have read the [Contribute
guide](https://www.reactiveui.net/contribute/index.html)
- [x] The PR title follows [Conventional
Commits](https://www.conventionalcommits.org/)
- [x] Tests cover this change, or the summary says why they do not
- [ ] New or changed public API has XML documentation

Author: glennawatson

src/Directory.Packages.props

@@ -69,7 +69,7 @@
   ItemGroup>
 
   <ItemGroup Label="ReactiveUI Source Generators">
-    <PackageVersion Include="ReactiveUI.SourceGenerators" Version="4.1.0"/>
+    <PackageVersion Include="ReactiveUI.SourceGenerators" Version="4.2.0"/>
   ItemGroup>
 
   <ItemGroup Label="Rx Ecosystem">