QT6 KDE Invent Repo has private/non-existent submodule

[fekhesk at pm.me]

At https://invent.kde.org/qt/qt/qt5 the `qtgrpc` submodule links to a
404 page, at least on the `dev` and `6.5.3` branches. Unsure if this is
correct, but it is causing errors when doing a recursive `git pull` of
the repository.
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