qt: Fix Window -> Minimize menu item on linux
qt: Fix Window -> Minimize menu item on linux

Summary:
On linux, the "Minimize" menu icon worked with using the `Ctrl + M` shortcut, but not with a click on the action in the menu. This fixes it.

Backport of [[https://github.com/bitcoin/bitcoin/pull/18549 | PR18549]]

Depends on D7894

Test Plan:
`ninja && ninja check`

Run `src/qt/bitcoin-qt` and check that the `Window -> Minimize` menu action works on linux, both via the keyboard shortcut and with a click in the menu.

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, jasonbcox

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D7895

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