Rename ImportCompeted into ImportCompleted

[jgcodes2020]

What does the pull request do?

This PR properly fixes issue #11960.

What is the current behavior?

The interface Avalonia.Rendering.Composition.ICompositionImportedGpuObject currently uses ImportCompeted for a Task that completes when the import is completed.

What is the updated/expected behavior with this PR?

Practically identical, except that name is fixed.

How was the solution implemented (if it's not obvious)?

ImportCompeted was renamed to ImportCompleted. A separate ImportCompeted property was added that simply fetches ImportCompleted. I have [Obsolete]d the ImportCompeted property to encourage people to switch over to the new spelling.

Checklist

• Added unit tests (if possible)? (N/A)
• Added XML documentation to any related classes?
• Consider submitting a PR to https://github.com/AvaloniaUI/Documentation with user documentation (GPU interop docs are currently not available yet)

Breaking changes

See Obsoletions/Deprecations section below.

Obsoletions / Deprecations

ICompositionImportedGpuObject.ImportCompeted is obsoleted (since it's a typo). The replacement is
ImportCompleted.

I have yet to discuss with the core team. (@grokys @kekekeks @danwalmsley)

Fixed issues

Fixes #11960

[jgcodes2020]

@dotnet-policy-service agree

[maxkatz6]

@jgcodes2020 thanks! We support netstandard2.0 and that's why you can't use default interface implementations. Instead, you need to implement this member independently.
Also couple of notes:

1. ICompositionGpuInterop should also be marked as not client implementabe
2. You will need to run "nuke ValidateApiDiff" from the repo root in the command line, so it will update breaking changes file.

[maxkatz6]

Thanks!
Looks good to me. I fixed CI build as well.

[avaloniaui-team]

You can test this PR using the following package version. 11.0.999-cibuild0037700-beta. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]