Fix HTTP2 => HTTP/1.1 downgrade bookkeeping

[MihaZupan]

Fixes #99401

Without the product change, the added test asserts here

runtime/src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/HttpConnectionPool.cs

Lines 1924 to 1925 in 4e86b1c

	Debug.Assert(_associatedHttp11ConnectionCount > _availableHttp11Connections.Count,
	$"Expected _associatedHttp11ConnectionCount={_associatedHttp11ConnectionCount} > _availableHttp11Connections.Count={_availableHttp11Connections.Count}");

Process terminated. Assertion failed.
Expected _associatedHttp11ConnectionCount=1 > _availableHttp11Connections.Count=1
    at System.Net.Http.HttpConnectionPool.ReturnHttp11Connection

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