### Pull Request Checklist
* [x] I have added Go unit tests or [Complement integration
tests](https://github.com/matrix-org/complement) for this PR _or_ I have
justified why this PR doesn't need tests
* [x] I have already signed off privately
This PR is in preparation for #3137 and removes the hard-coded username
validation (previously only dependent on `forceEmpty`).
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Co-authored-by: kegsay <7190048+kegsay@users.noreply.github.com>