chore: Suppress Xcode version validation for No-Mobile Solution

[Flash0ver]

Summary

Use property <ValidateXcodeVersion>false</ValidateXcodeVersion> for the NO_MOBILE Solution.

Remarks

When e.g. building on a macOS with Xcode 26.3 installed, the build produces error:

> Xamarin.Shared.Sdk.targets(2370,3): Error  : This version of .NET for macOS (26.2.10191) requires Xcode 26.2. The current version of Xcode is 26.3. Either install Xcode 26.2, or use a different version of .NET for macOS. See https://aka.ms/xcode-requirement for more information.

The current macOS/iOS version of .NET is built for Xcode 26.2, but has an identical SDK for Xcode 26.3.
See https://github.com/dotnet/macios/releases/tag/dotnet-10.0.1xx-xcode26.2-10217.

I believe that it is safe to suppress this error for local non-mobile development.

[github-actions]

Semver Impact of This PR

⚪ None (no version bump detected)

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.02%. Comparing base (5b676c4) to head (cdd79c5).
⚠️ Report is 27 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5054      +/-   ##
==========================================
+ Coverage   74.00%   74.02%   +0.02%     
==========================================
  Files         499      499              
  Lines       18048    18048              
  Branches     3512     3512              
==========================================
+ Hits        13356    13360       +4     
+ Misses       3838     3831       -7     
- Partials      854      857       +3     

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[Flash0ver]

question: "Correctness"

Keeping a development macOS machine up-to-date, which accompanies respective Xcode updates, my intention was to suppress the related error for the No-Mobile-Solution ... as a quick win.

I suppose we could "expand" this to the MAUI-including Solution (and Filtered Solutions respectively) as well, as long as we make sure that ValidateXcodeVersion=false is only set for local development, but definitely not for CI builds.

What do you think?

[jamescrosswell]

I think for the non-mobile slnf files, this makes sense. SDK users could be wanting to make contributions related to ASP.NET Core, without having to install Xcode etc.

We definitely want this check for the mobile filters though... and we just have to use something like xcodes to manage versions locally.