Replace JTF.RunAsync() with exception-safe wrappers

[mstv]

Follow-up to #11136 (and #11137 (comment))
inspired by d5b8f33

Proposed changes

• Replace JoinableTaskFactory.RunAsync() with exception-safe wrappers FileAndForget or InvokeAndForget, respectively
• Replace and remove GitUIExtensions.InvokeSync() and GitUIExtensions.InvokeAsync()

Screenshots

N/A

Test methodology

• exisiting tests

Please do not squash merge ❗

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✒️ I contribute this code under The Developer Certificate of Origin.

[RussKie]

The issue with this change it makes it close to impossible to mock the threading context. The current implementation allows us to replace the static ThreadHelper reference with an injected instance that provides JTF, but in the new implementation we won't be able to do that...

[mstv]

The injection of a JTF instance should be no big deal: The class TaskManager provides / encapsulates the JTF instance.
With the initial implementation, it would look like:

_taskManager.InvokeAndForget(SelectedDiff, () => SelectedDiff.ViewChangesAsync(...));

With another argument added to the extension method, it can be written as before (preferred):

SelectedDiff.InvokeAndForget(() => SelectedDiff.ViewChangesAsync(...), _taskManager);

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Furthermore, I should feed every already available CancellationToken to InvokeAndForget(), too - but in another follow-up PR:

CancellationToken cancellationToken = _viewChangesSequence.Next();
SelectedDiff.InvokeAndForget(() => SelectedDiff.ViewChangesAsync(..., cancellationToken), _taskManager, cancellationToken);

[mstv]

On hold: The test execution will improve / work again after merging the 3 today's PRs.

[mstv]

needs #11162