fix(register.rego): increase allowed username length to 64

[AlexanderBabel]

This PR addresses the issue #2454

Reason

The current username limit of 14 characters feels arbitrary and I did not find any reason for that limit.

I checked the following components:

• MAS Database: The column username in users is of type text and hence not limiting the username length.
• Synapse: To test a long username, I created a new account on matrix.org with a username that is 64 characters long. It worked without any issues.

[sandhose]

Thanks for this! I'm not sure it's even worth having this limit at all, but in the meantime bumping that to 64 characters makes sense