Capital case username can't be used during installation

Summary

Using capital username during installation setup will have the error message "Invalid username. Username mustn't be blank, have trailing or leading space and must have no more length than 255 characters."

Steps to Reproduce

1. Install Bamboo 6.6+
2. When setting up Administrator user, fill in capital case username like "Admin"

Expected Results

The admin user is created successfully

Actual Results

It will have error message "Invalid username. Username mustn't be blank, have trailing or leading space and must have no more length than 255 characters." under the Username field

Notes

After finishing the installation with lower case username, it is possible to create capital case username at Administration >> Security >> Users - it will just auto convert to lower case