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/db/models/group.rs

@@ -104,7 +104,6 @@ impl Group {
             "id": self.uuid,
             "organizationId": self.organizations_uuid,
             "name": self.name,
-            "accessAll": self.access_all,
             "externalId": self.external_id,
             "collections": collections_groups,
             "object": "groupDetails"
@@ -574,26 +573,6 @@ impl GroupUser {
         }
     }
 
-    pub async fn delete_by_group_and_member(
-        group_uuid: &GroupId,
-        member_uuid: &MembershipId,
-        conn: &DbConn,
-    ) -> EmptyResult {
-        match Membership::find_by_uuid(member_uuid, conn).await {
-            Some(member) => User::update_uuid_revision(&member.user_uuid, conn).await,
-            None => warn!("Member could not be found!"),
-        }
-
-        conn.run(move |conn| {
-            diesel::delete(groups_users::table)
-                .filter(groups_users::groups_uuid.eq(group_uuid))
-                .filter(groups_users::users_organizations_uuid.eq(member_uuid))
-                .execute(conn)
-                .map_res("Error deleting group users")
-        })
-        .await
-    }
-
     pub async fn delete_all_by_group(group_uuid: &GroupId, org_uuid: &OrganizationId, conn: &DbConn) -> EmptyResult {
         let group_users = GroupUser::find_by_group(group_uuid, org_uuid, conn).await;
         for group_user in group_users {