Improve `xdg-desktop-portals` processes termination since
`xdg-desktop-portals` is 19 characters long.
From `killall(1)` man page:
> -e, --exact
>
> Require an exact match for very long names. If a command name is longer
> than 15 characters, the full name may be unavailable (i.e. it is swapped
> out). In this case, `killall` will kill everything that matches within
> the first 15 characters. With -e, such entries are skipped. `killall`
> prints a message for each skipped entry if -v is specified in addition
> to -e.
XDPH doesn't need ** keyword, it means portage will pull in the latest git version of XDPH available, which at the time of writing the wiki may have been required, but is not necessary now.
* Update Gentoo installation instructions for XDPH
* Changed the suggested overlay from useless-overlay to GURU.
* Bumped qt versions in the docs from 6.4.0 to 6.4.3 since 6.4.0 is not available anymore.
* Remove dependency on specific Qt versions for the Gentoo instructions
Now, the instructions suggest to just unmask the whole package instead of specific versions.
