Hot fix existing username doesn't match regrex

[lenguyenthanh]

This revert changes from 0e5b913#diff-6f8de4a74a854935ffe91ec38c7bb20004348e5915c91ad090750c5e4e62cdfeL78

Fix: https://discord.com/channels/280713822073913354/1240568041377370112/1240568041377370112

[ornicar]

    // what existing usernames are like
    val historicalRegex     = "(?i)[a-z0-9][a-z0-9_-]{0,28}[a-z0-9]".r

if there exist usernames that don't match that regex, then it's the regex we should be changing.

[ornicar]

30ec1a1