chore: Move MAUI sample solution file under samples

[filzrev]

This PR move BenchmarkDotNet.Maui.slnx file under samples directory, and modify related paths.

Background
After Verify.Xunit package version updated to 31.12.5.

Solution-Discovery feature is automatically enabled.
And following warning message are recorded on CI.

 /Users/runner/.nuget/packages/verify/31.12.5/buildTransitive/Verify.props(60,5): warning : Multiple solution files found. Unable to auto-discover SolutionDir and SolutionName.

And It seems tests randomly failed to build auto-generated project with following errors.

 /Users/runner/.nuget/packages/verify/31.12.5/buildTransitive/Verify.props(70,5): warning : No solution files found. Unable to auto-discover SolutionDir and SolutionName. Verify searches for .slnx and .sln files in the project directory '/Users/runner/work/BenchmarkDotNet/BenchmarkDotNet/tests/BenchmarkDotNet.IntegrationTests/bin/Release/net8.0/BenchmarkDotNet.IntegrationTests-Dry-369', parent directory, and parent's parent directory. To resolve this, either add a solution file to one of these locations, or explicitly set SolutionDir and SolutionName via command line: /p:SolutionDir="C:\Path\To\Solution\" /p:SolutionName="MySolution" [/Users/runner/work/BenchmarkDotNet/BenchmarkDotNet/tests/BenchmarkDotNet.IntegrationTests/bin/Release/net8.0/BenchmarkDotNet.IntegrationTests-Dry-369/BenchmarkDotNet.Autogenerated.csproj::TargetFramework=net8.0]
/Users/runner/work/BenchmarkDotNet/BenchmarkDotNet/.dotnet/sdk/10.0.103/Roslyn/Microsoft.CSharp.Core.targets(84,5): error : Unhandled exception. System.MissingMethodException: Method not found: 'Microsoft.CodeAnalysis.CSharp.Symbols.MethodSymbol Microsoft.CodeAnalysis.CSharp.Symbols.PropertySymbolExtensions.GetOwnOrInheritedSetMethod(Microsoft.CodeAnalysis.CSharp.Symbols.PropertySymbol)'. 

I don't know Verify.Xunit package update causing this MissingMethodException thought.
To isolate the problem, move BenchmarkDotNet.Maui.slnx file to separate directory.

[filzrev]

warning : Multiple solution files found. is disappeared.
Though, following warning is still generated 6 times on browser-wasm related dotnet publish timing.

Verify.props(70,5): warning : No solution files found. Unable to auto-discover SolutionDir and SolutionName.

[timcassell]

What is its purpose? Can we disable it?

[filzrev]

What is its purpose? Can we disable it?

As far as I've confirmed.

It seems there is no options to disable Solution-Discovery feature that is added from v31.10.0 or later.
And it can't find reasons why this feature is added on Verify.net.

[filzrev]

Following warning is suppressed by setting SolutionDir at tests/Directory.Build.props.

warning : No solution files found. issue is resolved by explicitly setting SolutionDir at tests/Directory.Build.props.

[SimonCropp]

verify uses the solution to do this scrubbing https://github.com/VerifyTests/Verify/blob/main/docs/scrubbers.md#directory-scrubbers

the assumption is that most people consuming verify would actually like to know when one of the scrubbers cant work, hence the warning

[timcassell]

Thanks (disregard my previous comment)