Update Shepherd to 9.1.1

Problem/Motivation

Drupal 9.5.x ships with Shepherd 9.1.0, but 9.1.1 is out: https://github.com/shipshapecode/shepherd/blob/master/CHANGELOG.md#v911-...

Steps to reproduce

Proposed resolution

yarn upgrade shepherd.js
yarn vendor-update

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet