Dropdown opens under modals

[Pixelshaped]

Since a few months, and following these commits on the Bootstrap5 project:

• 2021-03-06: https://github.com/twbs/bootstrap/commit/548be2ed6604ddfc8488cd4a793c6271c2caf485#diff-108a57a50d5ebe203f447ef7bf1e5e53826bdb82baab2baa2b41b4158cdd4d09
• 2021-06-25: https://github.com/twbs/bootstrap/commit/45d26de72817b295c5f94c8426354fd5b7d0a1f9#diff-108a57a50d5ebe203f447ef7bf1e5e53826bdb82baab2baa2b41b4158cdd4d09

The default z-index for a modal is not '1050' anymore, but '1055'. This means when using select2 inside a modal the dropdown will open under the modal.

z-index is currently specified in the main select2 css, not in this project. As this regards Bootstrap5 only, I think this modification should probably be made in this project.