Username regex mismatch between signup and change processes

[chloe-arkwright]

Please confirm the following.

• [x] I checked the existing issues for duplicate problems
• [x] I have tried resolving the issue using the support portal

What browsers are you seeing the problem on?

Firefox

Describe the bug

The regex validation for the user's username appears to be different between the signup page and the profile page where you can change your username.

For example, this user exists with a period (.) in their username but if you try to change your username to contain a period then it will fail the regex validation.

https://modrinth.com/user/satan.son_

I can confirm that my account also had a period in the username before I wanted to change it slightly.

Steps to reproduce

1. Go to profile page
2. Try to update username to one containing a period
3. See it fails validation
4. Go to sign up page
5. Try to create an account with the same username
6. See it passes validation.

Expected behavior

Regexs match either by disallowing periods or allowing usernames to be changed to allow periods.

Additional context

No response