Restore username validation to disallow dots, for now (mastodon#6863)

Usernames with dots in them do not work with routes, because the dot usually separates the desired page format (e.g. json). I don't want to mess with changing route constraints for this patch release.

app/models/account.rb

@@ -69,7 +69,7 @@ class Account < ApplicationRecord
   validates :username, uniqueness: { scope: :domain, case_sensitive: true }, if: -> { !local? && will_save_change_to_username? }
 
   # Local user validations
-  validates :username, format: { with: /\A#{USERNAME_RE}\z/i }, length: { maximum: 30 }, if: -> { local? && will_save_change_to_username? }
+  validates :username, format: { with: /\A[a-z0-9_]+\z/i }, length: { maximum: 30 }, if: -> { local? && will_save_change_to_username? }
   validates_with UniqueUsernameValidator, if: -> { local? && will_save_change_to_username? }
   validates_with UnreservedUsernameValidator, if: -> { local? && will_save_change_to_username? }
   validates :display_name, length: { maximum: 30 }, if: -> { local? && will_save_change_to_display_name? }