Update error message for disallowed username characters

[marcarmengou]

This PR replaces illegal characters with disallowed characters, removing unnecessarily alarmist language for the user.

https://core.trac.wordpress.org/ticket/65084

[github-actions]

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Props marc4.

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[github-actions]

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[nikunj8866]

Hey @marcarmengou, thanks for the contribution!

As per the CONTRIBUTING.md of this repository, pull requests must include a link to a corresponding ticket on core.trac.wordpress.org:

Pull requests are accepted as long as there is a corresponding Trac ticket. Please include a link to a pre-existing ticket on https://core.trac.wordpress.org/ with every pull request.

Could you please:

1. Open a new ticket at https://core.trac.wordpress.org/ describing the rationale for changing illegal characters → disallowed characters (worth noting this also affects existing translations)
2. Update this PR description with the Trac ticket URL
3. Add the has-patch keyword on the Trac ticket and link back to this PR

For reference: GitHub Pull Requests for Code Review – Make WordPress Core

Happy to help if you have questions about the process!

[nikunj8866]

LGTM!