Date of birth in user settings

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Is your feature request related to a problem? Please describe.
Birthdays must be maintained manually by a person in a user account-dependent calendar. The calendar must then be shared with everyone in the instance and is in addition to the existing birthday calendar.

Describe the solution you'd like
The feature that users of the Nextcloud instance can enter their date of birth in the Personal Information section. This could (if the user so desires) be displayed on the user page. Other apps such as the calendar app, for example, could then transfer the birth dates to the existing birthday calendar (if this function is activated on the respective Nextcloud instance).

Additional context
Confusion among users why there are two calendars for birthdays.

Start at the Personal Information section and trace how profile fields are stored and exposed on the user page. Then review the existing birthday calendar path and determine the instance setting, privacy behavior, and calendar-app integration described in the issue. Done means the feature scope and behavior are defined and implemented consistently across these entry points.