fix(signup): remove default 20-years-ago birth date assumption

[Puspak29]

Summary

The signup form was automatically filling in a birth date 20 years before today. This incorrectly assumed every new user was 20 years old.

Now, the birth date field starts blank, and users must select their actual date of birth before continuing.

UI Change

Before:
Birth date was automatically filled with a date 20 years ago.

After:
Birth date starts empty, and the user must select a date before continuing or creating an account.

Fixes #10935