devel/kf5-knewstuff: add upstream backport requests

Recently KDE experienced a high load on their servers due to
non-CDN urls in knewstuff & friends.

This adds upstream commits

• Engine: Ensure we are not using the wrong ProvidersUrl

		https://invent.kde.org/frameworks/knewstuff/-/commit/c8165b7a0d622e318b3353ccf257a8f229dd12c9

• Add http2 to KNewStuff requests (was only added for the third

	  one to apply easily)
		https://invent.kde.org/frameworks/knewstuff/-/commit/9a98fc12d8b049ef2df73ad46bf628ea99e7c8a5

• Add conditional cache preference to http requests

		https://invent.kde.org/frameworks/knewstuff/-/commit/e1c6f2bf383876a31cd3e3f9e6edcaa19dc0a7dd