Remove use of term "special characters" in username validation
Username validation uses imprecise and non-inclusive language. Specifically, it uses the term "special characters", which is ambiguous and sometimes redundant, when it is accompanied with a more specific error.
See #20184 (comment 1609251707) for more context.
The current errors a user might see include:
Username must not start or end with a special character and must not contain consecutive special characters.
Username can contain only letters, digits, '_', '-' and '.'. Cannot start with '-' or end in '.', '.git' or '.atom'.
The problematic error message is the first one. This is also true of group names but it's particularly problematic with usernames because the use of the word "special" seems to refer to the use of accented and non-Latin characters, which are not supported.
Proposal
Remove this error message entirely and avoid the use of the term, "special characters". Instead, be specific about what is wrong with the username, exemplified in the second example.
Implementation Guide
Should be straightforward. Look for the text above in the codebase and change instances to match the second example.