Update template for bootstrap 5 to fix "Add slots" dropdown menu not working in Moodle 5

[monnardj]

Moodle 5.0 has migrated to booststrap 5. As a consequence, the dropdown menu "Add slots" in the scheduler activity is no longer working. This is because data attributes have been namespaced. data-toggle="dropdown" is now data-bs-toggle="dropdown". This pull request adapts the template with the new attribute name.

[danielneis]

Thank you for the patch, I can confirm this works. Hopefully the maintainer will merge it anytime...