refactor(builder)!: remove BuilderMixins members shadowed by IReactiveUIBuilder (#4498)

## Summary

**`BuilderMixins` no longer repeats members that `IReactiveUIBuilder`
already declares.**

- **30 extension members are removed.** Each had the same name and a
compatible parameter list as an `IReactiveUIBuilder` member, so
`builder.X(...)` always called the interface member and the extension
was reachable only as `BuilderMixins.X(builder, ...)`.
- **`ForCustomPlatform` now has one behaviour.** The removed extension
ran the platform registrations at once. The interface member, which
every `builder.ForCustomPlatform(...)` call already used, runs them at
build.
- **The members that add something stay.** `RegisterViews`,
`WithViewModule`, `WithConverters`, `UsingSplatBuilder` and
`BuildApp` on `IAppBuilder` are unchanged.

## Why

**The removed extensions were dead public API, and one of them disagreed
with the member it shadowed.**

- C# only looks for an extension member when no instance member applies.
For a receiver typed `IReactiveUIBuilder` or `ReactiveUIBuilder` the
instance member always applies, so these extensions were never chosen.
`BuilderMixins.ForCustomPlatform(builder, ...)` registered the platform
services straight away, while `builder.ForCustomPlatform(...)` deferred
them to build.

Closes #4488

## Breaking changes

**These `BuilderMixins` members are removed. Call the
`IReactiveUIBuilder` member of the same name instead.**

- `WithTaskPoolScheduler` (both overloads), `WithMainThreadScheduler`
(both overloads), `WithRegistrationOnBuild`, `WithRegistration`,
`WithViewsFromAssembly`, `WithPlatformModule`, `UsingSplatModule`.
- `WithConverter` (all four overloads), `WithFallbackConverter` (both
overloads), `WithSetMethodConverter` (both overloads),
`WithConvertersFrom`.
- `ForCustomPlatform`, `ForPlatforms`, `WithMessageBus` (all three
overloads), `ConfigureViewLocator`, `ConfigureSuspensionDriver`.
- `RegisterViewModel`, `RegisterConstantViewModel`,
`RegisterSingletonViewModel`, `RegisterView`,
`RegisterSingletonView`.
- Code written as `builder.X(...)` compiles and behaves as before. Only
the static form `BuilderMixins.X(builder, ...)` breaks. A caller that
used `BuilderMixins.ForCustomPlatform(builder, ...)` and relied on the
registrations existing before `Build()` must call `WithRegistration` for
them instead.

## How this was verified

**A Roslyn binding probe calls every former `BuilderMixins` member by
extension syntax on `IReactiveUIBuilder`, `ReactiveUIBuilder` and
`IAppBuilder` receivers; every call compiles and binds to the same
method before and after the change.**

- Tests that called the removed members through the static form now call
the interface members. The `ForCustomPlatform` test also checks that the
registrations appear only at build. The documentation examples,
including the `rxappbuilder` page, build and run against the change.

## Notes for the reviewer

**The change is deletions in
`src/ReactiveUI.Shared/Mixins/BuilderMixins.cs`, shared by the lean and
`.Reactive` flavours.**

- `UsingSplatBuilder` stays because `IReactiveUIBuilder` has no such
member, so the extension is what an `IReactiveUIBuilder`-typed caller
gets. `BuildApp` stays because its receiver is Splat's `IAppBuilder`.
- `WithViewsFromAssembly` keeps its `RequiresUnreferencedCode`
annotation on the interface member and on `ReactiveUIBuilder`.
- `ReactiveUI` and `ReactiveUI.Reactive` have no committed public API
baselines on `main`, so there is no baseline to update.
- The test edits are mechanical: `BuilderMixins.X(builder, ...)` becomes
`builder.X(...)`, and the null-builder checks for removed members are
gone.

## 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
- [x] New or changed public API has XML documentation

Author: glennawatson