fix warnings on PHP 8, re #160

controllers/solutions.php

exercises/Exercise.php

exercises/algebraq/parser.php

exercises/cloze_exercise.php

exercises/sc_exercise.php

exercises/tb_exercise.php

views/exercises/cloze_exercise/edit.php

views/exercises/flexible_input.php

views/exercises/lt_exercise/edit.php

views/exercises/print_exercise.php

views/solutions/assignments_list.php

views/solutions/participants_overview.php