fix php8 warnings, fixes #1679

app/controllers/calendar/calendar.php

app/controllers/calendar/schedule.php

app/controllers/contact.php

app/controllers/course/details.php

app/controllers/course/members.php

app/controllers/elearning.php

app/controllers/news.php

app/controllers/profilemodules.php

app/controllers/search/archive.php

app/controllers/settings/statusgruppen.php

app/views/api/authorizations/index.php

app/views/calendar/schedule/index.php

app/views/course/details/index.php

app/views/course/feedback/index_for.php

app/views/course/members/autor_list.php

app/views/course/plus/index.php

app/views/course/wizard/steps/studyareas/_node.php

app/views/files/_fileref_tr.php

app/views/files/_files_thead.php

app/views/files/_overview.php