DiscoverWindow.qml: Port message OverlaySheet to Dialog and make it friendlier

Currently the passiveMessage dialog in DiscoverWindow is a bit too in-your-face with the errors, causing sadness and confusion with users. It also used Kirigami.OverlaySheet even it is more of a dialog.

This MR makes it use Kirigami.Dialog features and hides the technical details behind a button, plus gives it a friendlier message.

Before	After

BUG: 468657 BUG: 482551 FIXED-IN: 6.2.0