User ID length limit is not correct (should be 255 for the whole User ID, not just the local part)

[matrixbot]

This issue was originally created by @reivilibre at matrix-org/matrix-authentication-service#2861.

https://github.com/matrix-org/matrix-authentication-service/blob/121966ccceaf74b1c5728d5521299eac3ff9346d/crates/handlers/src/graphql/mutations/user.rs#L276

https://spec.matrix.org/v1.10/appendices/#user-identifiers

The length of a user ID, including the @ sigil and the domain, MUST NOT exceed 255 characters.

Other than that, it seems that + chars are not allowed even though the spec says they should be