Add PHP8.3 support

[ChristophWurst]

Describe the task

https://php.watch/news/2023/08/php83-rc1-released

Affected components

Calendar, Contacts, Mail

To do

• [x] https://github.com/nextcloud/mail/issues/8918
• [x] https://github.com/nextcloud/contacts/issues/3641
• [x] https://github.com/nextcloud/calendar/issues/5507

Additional context

• https://github.com/nextcloud/server/pull/40630

[ChristophWurst]

It's done