[ChangeLog][QtNetwork][QNetworkRequest] Added
QNetworkRequest::Http2CleartextAllowedAttribute which controls whether
HTTP/2 cleartext (h2c) is allowed or not. The default is false. This
replaces the QT_NETWORK_H2C_ALLOWED environment variable.
Task-number: QTBUG-98642
Change-Id: I43ae1cc671788f6d2559cd316f6667b412c8e75e
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>