Fix expect 100 continue logic in Kestrel to avoid doing sync over async

[davidfowl]

Describe the bug

Though this doesn't cause any problems in practice (we believe), there's a code cleanup that needs to happen around expect 100 continue handling in Kestrel. The code paths ProduceContinue are called from are all asynchronous, yet the method is synchronous and does sync over async. Lets fix that finally.

Here's the code

aspnetcore/src/Servers/Kestrel/Core/src/Internal/Http/HttpProtocol.cs

Line 909 in 333134c

public void ProduceContinue()

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[davidfowl]

@wtgodbe assigned this to you for the next preview.