fix(registration): enforce the strict user ID grammar

previously, !is_historical was used, but this caused user IDs that are no longer allowed under the current spec version's historial grammer to be allowed in registration

(cherry picked from commit 3248efbe4b)

src/api/client_server/account.rs

@@ -25,7 +25,7 @@ const RANDOM_USER_ID_LENGTH: usize = 10;
 /// Checks if a username is valid and available on this server.
 ///
 /// Conditions for returning true:
-/// - The user id is not historical
+/// - The user id must be valid according to the strict grammar
 /// - The server name of the user id matches this server
 /// - No user or appservice on this server already claimed this username
 ///
@@ -40,7 +40,8 @@ pub async fn get_register_available_route(
     )
     .ok()
     .filter(|user_id| {
-        !user_id.is_historical() && user_id.server_name() == services().globals.server_name()
+        user_id.validate_strict().is_ok()
+            && user_id.server_name() == services().globals.server_name()
     })
     .ok_or(Error::BadRequest(
         ErrorKind::InvalidUsername,
@@ -92,7 +93,7 @@ pub async fn register_route(body: Ruma<register::v3::Request>) -> Result<registe
             )
             .ok()
             .filter(|user_id| {
-                !user_id.is_historical()
+                user_id.validate_strict().is_ok()
                     && user_id.server_name() == services().globals.server_name()
             })
             .ok_or(Error::BadRequest(