[ical4j 4.x] Test with Posteo

[rfc2822]

Created from checklist item in #241.

Original text:

Posteo

[cketti]

Tested with an Android 14 emulator.

• Event added on server syncs to device
• Event added on device syncs to server
• Event changed on server syncs to device
• Event changed on device syncs to server
• Event deleted on server syncs to device
• Event deleted on device syncs to server
• Recurring event with exception syncs fine both ways
• Task added on device syncs to server
• Task changed on device syncs to server
• Task deleted on device syncs to server