[Enhancement]: configure minimal age for registration

Description

Just like Loops, allow instance admins to define a custom minimal age for registration. This could be using an environment variable, and/or using the admin panel.

Then adapt the files resources/views/site/help/safety-tips.blade.php (line 19) and resources/views/auth/curated-register/partial/step-1.blade.php(lines 74-77 and subsequent logic) to make them consistent.

Today, both files are hard-coded with ages which no longer reflect current legislation, nor does it take into account all countries. It should be the responsibility of the instance admin to define and enforce the minimum age limit.

Use-case

Comply with server country legislation and with the admin rules for a minimal age for resgistration.

Acknowledgements

Yes