Explicitly call QCoreApplication::exit()

> In Qt 5, QCoreApplication::quit() was equivalent to calling QCoreApplication::exit().
> This just exited the main event loop.
> In Qt 6, the method will instead try to close all top-level windows by posting a close event.
> The windows are free to cancel the shutdown process by ignoring the event.
> Call QCoreApplication::exit() to keep the non-conditional behavior.

Kate already does its own cleanup and close handling when the event loop
exits, now leading to double prompts when Qt sends a close event and then
Kate tries to close them. Instead, just call `exit()`.

CCBUG: 469690


(cherry picked from commit 1949852a)