Improve handling of "Date of Birth" (dob) infotext in user profile plugin

### Issue
Currently, the user - profile plugin uses a separate spacer field to display an info text for the "date of birth" (dob) field.
This makes it necessary to add some specific codechecks for when that spacer field should be shown or hidden. Like done in #4043.

### Proposal
This PR moves the dob field and the infotext into a custom formfield and removes the special code for the dob_spacer field.

### Questions
While trying this solution I wondered why we even have this info text separated. I think best place would actually be the tooltip, which is exactly meant for that kind of information.
The plugin currently overrides the description set in the profile.xml with a hardcoded and quite useless text. I'm not sure why that is but I guess it's for historical reasons, maybe before there was a separate profile.xml file?

Does anyone know why this was done like this?