Revert "Merge branch 'remove-unused-css-from-mr-24724' into 'master'"

What does this MR do?

The CSS class d-flex-center that was removed is actually used and helpful.

Please note!

Here is the corresponding EE MR https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/10431 that reverts the EE changes.

Why was d-flex-center removed?

According to the commit message (https://gitlab.com/gitlab-org/gitlab-ce/commit/d030b0253d43b24991987b3dc149081f3b4dbbb7) it was incorrectly assumed that this is the same as BS4 align-items-center. This is not the case. d-flex-center is a helpful composite class of d-flex justify-content-center align-items-center. Without this, the vertical and horizontal alignment of some elements are off.

Without d-flex-center (on master)	With d-flex-center

This is also needed by some other MR's:

• https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/10056/diffs#effbf7c30fb2867a5e404e53ea90966db6dd24db_0_18

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer