Add .NET 8 target (Avalonia 11.1)

[maxkatz6]

What does the pull request do?

There is a number of features that we need from .NET 8 SDK and targets.
Primarily:

1. Keep mobile support up to date. Xamarin team actively drops support for .NET TFM versions, and right now it's not possible to build mobile app with .NET 6 TFM. .NET 7 still works, but this PR is going one step ahead.
2. UnsafeAccessor which is going to be helpful with SkiaSharp 3.0 compatibility
3. Up-to-date SDK analyzers.
4. Other potential optimizations available only on .NET 8 SDK. Though we still can't use .NET 8 only features in our Public API (like, DateOnly primitives), but it can go to Avalonia.Labs.

And also, we are likely to drop .NET 6 in Avalonia 12.0 (while still likely to support .NET Standard 2.0). Targeting .NET 8 is a preparation before that.

[avaloniaui-bot]

You can test this PR using the following package version. 11.1.999-cibuild0044613-beta. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[workgroupengineering]

Instead of directly using TargetFramework net8.0 for each project, add to Directory.Build.props a property like

<PropertyGroup>
  <AvaloniaDefaultTargetFramework Condition="'$(AvaloniaDefaultTargetFramework)' == ''">net8.0</AvaloniaDefaultTargetFramework>
</PropertyGroup>

eg. using in packages/Avalonia/Avalonia.csproj

  <PropertyGroup>
      <TargetFrameworks>$(AvaloniaDefaultTargetFramework);netstandard2.0</TargetFrameworks>
      <PackageId>Avalonia</PackageId>
  </PropertyGroup>

eg: samples/MobileSandbox.Browser/MobileSandbox.Browser.csproj

 <PropertyGroup>
    <TargetFramework>$(AvaloniaDefaultTargetFramework)-browser</TargetFramework>
    <RuntimeIdentifier>browser-wasm</RuntimeIdentifier>
    <Nullable>enable</Nullable>
    <EmccTotalMemory>16777216</EmccTotalMemory>
 </PropertyGroup>

For example, when you switch to net9.0 targte you will only need to change it in one point.

[maxkatz6]

Yeah, we discussed that internally.
I had an idea for DotNetCurrent=net8.0, DotNetPrevious=net6.0, DotNetFramework=net461, DotNetStandard=netstandard2.0.

But we couldn't find if it really would make anything better. It does make it easier to update version, but at the same time harder to quickly check csproj version for users no used to the repo. And in general, it's not like we change these versions often. Next time probably will happen around .NET 10 haha.

[avaloniaui-bot]

You can test this PR using the following package version. 11.1.999-cibuild0044637-beta. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[avaloniaui-bot]

You can test this PR using the following package version. 11.1.999-cibuild0044701-beta. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[kekekeks]

Can we have <LegacyTargetFrameworks Condition="'$(AvaloniaSkipLegacyTargetFrameworks)' != 'True'>net6.0</LegacyTargetFrameworks> so building for net6.0 can be disabled during development?

[maxkatz6]

@kekekeks it would complicate msbuild logic quite a bit. Will think about it. It would also add DotNetCurrent/DotNetPrevious props anyway, which is fine.

[kekekeks]

it would complicate msbuild logic quite a bit

Why?
It would be something like this in common props:

<DotNetCurrentFramework>net9.0</DotNetCurrentFramework>
<DotNetCompatibilityFrameworks>net8.0;net6.0<DotNetCompatibilityFrameworks>
<CommonTargetFrameworks>$(DotNetCurrentFramework);netstandard2.0</CommonTargetFrameworks>
<CommonTargetFrameworks Condition="'$(AvaloniaSkipLegacyTargetFrameworks)' != 'True'>$(CommonTargetFrameworks);$(DotNetCompatibilityFrameworks)</CommonTargetFrameworks>

and <TargetFrameworks>$(CommonTargetFrameworks)</TargetFrameworks> in csproj files.

[workgroupengineering]

If you decide to add properties, it may be best to add the Avalonia prefix to the properties to try to avoid conflicts.

[maxkatz6]

It would be something like this in common props:

So, how would we add "-ios", "-android", "-browser" suffix?
It would require quite a bit of duplication. Possible, but exactly what I meant before by quite complicated.

[kekekeks]

OK, it's out of the scope of this PR anyway.

[avaloniaui-bot]

You can test this PR using the following package version. 11.1.999-cibuild0045118-beta. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[avaloniaui-bot]

You can test this PR using the following package version. 11.1.999-cibuild0045301-beta. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[avaloniaui-bot]

You can test this PR using the following package version. 11.1.999-cibuild0045371-beta. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[avaloniaui-bot]

You can test this PR using the following package version. 11.1.999-cibuild0045525-beta. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[avaloniaui-bot]

You can test this PR using the following package version. 11.1.999-cibuild0045700-beta. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]