Warning fixes: spring wave

[MrJul]

It's spring, warnings again are blooming, now's the time for fixing!

Noteworthy changes:

• Base: remove internal Obsolete attributes on ValueStore (with @grokys' approval).
• RenderTests: updated ImageSharp to 2.1.8 (only used in tests, but has known vulnerabilities).
• Browser: only enable PThread if WasmEnableThreads is true.
• ControlCatalog.Desktop: reference Win32+Skia directly to avoid bringing incompatible packages to net461.

[avaloniaui-bot]

You can test this PR using the following package version. 11.2.999-cibuild0048509-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[maxkatz6]

We need to unify how these pollyfils are referenced. Somewhere it's part of Avalonia.Base + internals visible to, and somewhere it's a shared file.

[maxkatz6]

Not sure what's better. But shared file is more annoying when you already have internals visible to enabled.

[MrJul]

We can't really unify, as much as I'd like that.

As you mentioned, the InternalsVisibleTo mandates that we use the types from Avalonia.Base if we don't want spurious warnings for conflicting attributes (like the ones this PR fixes). But a few projects, such as Avalonia.Designer.HostApp aren't dependent of Avalonia.Base, so we have to use the shared files instead.

Should be resolved if/when we drop netstandard2.0, hopefully before 2030 ;)

[maxkatz6]

:)

[avaloniaui-bot]

You can test this PR using the following package version. 11.2.999-cibuild0048539-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]