Better UX for the lowercase username requirement on registration

### Your use case

#### What would you like to do?

Remove confusion around lowercase username

#### Why would you like to do it?

While on-boarding 10 communities on their own homeservers, I noticed this is a big hurdle for many. Either the warning message is unclear or the requirement is unexpected.

#### How would you like to achieve it?

Just lowercase the name, don't warn, don't ask. Or issue a more specific warning message.

### Have you considered any alternatives?

_No response_

### Additional context

_No response_