snap: Migrate to core24

[bjorn]

When this works, look into updating from Qt 5 to Qt 6 as well.

I went through https://snapcraft.io/docs/migrate-core24 but there didn't seem to be any relevant actions required.

[bjorn]

Seems like it might be a good idea to just update to core24 for now and figure out the Qt 6 update later.

:: Detecting Qt installations...
:: Found Qt at '/usr/bin/qmake'.
:: Found Qt at '/bin/qmake'.
:: Setting up Qt at '/usr/bin/qmake'...
:: WARNING: Error setting up Qt for '/usr/bin/qmake': Error: The qmake executable '/usr/bin/qmake' failed with exit code 1.
:: Setting up Qt at '/bin/qmake'...
:: WARNING: Error setting up Qt for '/bin/qmake': Error: The qmake executable '/bin/qmake' failed with exit code 1.

I wish there was at least some kind of reason printed...

[bjorn]

Seems like it might be a good idea to just update to core24 for now and figure out the Qt 6 update later.

Opened #4068 for updating to Qt 6.