Decreases z-index of select2 to a lower number of our navigation bar

What does this MR do?

Decreases z-index of select2 to a lower number of our navigation bar

We also need to reduce z-index of select2-drop-mask:

1. https://gitlab.com/gitlab-org/gitlab-ce/issues/36160#note_43698982
2. https://gitlab.com/gitlab-org/gitlab-ce/issues/36160#note_43699092

Original MR: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/14768

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Review
  • Has been reviewed by Frontend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes #36160 (closed)