Current list of default action ids:

"update_user_locale", "CONFIGURE_TOTP", "delete_account", "VERIFY_EMAIL", "webauthn-register-passwordless", "webauthn-register", "VERIFY_PROFILE", "delete_credential", "UPDATE_PASSWORD", "UPDATE_PROFILE"

I got the list by adding every action manually to a user in the admin console, then doing the GET request to /admin/realms/{realm}/users/{user-uuid} to retrieve the JSON representation of the actions in the requiredActions field.