Remove legacy API endpoints and compatibility code dropped upstream (#7809)

Remove endpoints, response fields, feature flags and compatibility code
that upstream Bitwarden has removed and that no current client uses:

- Legacy POST /identity/accounts/register and POST /api/accounts/prelogin;
  register/finish is rate limited like the other unauthenticated endpoints.
- Legacy organization routes: PUT groups/{id}/users, PUT policies/{type}/vnext,
  PUT users/{id}/reset-password, GET billing/metadata, and the POST aliases of
  the organization, collection, group and member PUT/DELETE routes.
- POST /two-factor/disable (PUT stays).
- ResetMasterPassword, captchaBypassToken, the legacy registration name field,
  userIsManagedByOrganization/managedByOrganization and accessAll.
- The automatic email 2FA code for clients older than 2025.5.0.
- DUO_USE_IFRAME and the Duo Traditional Prompt code.
- Experimental client feature flags that no client reads anymore.
- Web vault CSS selectors for web vaults older than 2025.5.1.

Also accept the PascalCase fields Android 2026.9.0+ sends on registration
and password change, and align a few account, login/2FA and organization
checks with upstream.

Adds tests for the registration body formats.

src/api/admin.rs

@@ -508,6 +508,7 @@ async fn enable_user(user_id: UserId, _token: AdminToken, conn: DbConn) -> Empty
 async fn remove_2fa(user_id: UserId, token: AdminToken, conn: DbConn) -> EmptyResult {
     let mut user = get_user_or_404(&user_id, &conn).await?;
     TwoFactor::delete_all_by_user(&user.uuid, &conn).await?;
+    Device::clear_twofactor_remember_by_user(&user.uuid, &conn).await?;
     two_factor::enforce_2fa_policy(&user, &ACTING_ADMIN_USER.into(), 14, &token.ip.ip, &conn).await?;
     user.totp_recover = None;
     user.save(&conn).await