Downgrade drupal/bootstrap to 3.26

Problem/Motivation

Make sure we use the latest version of Bootstrap.

We updated Open Social to Drupal 9.5.3 (PR)
We need to upgrade Bootstrap to at least 3.26 as stated in PR.

Proposed resolution

This uses:
https://www.drupal.org/project/bootstrap/releases/8.x-3.26 →