Taxonomy: Fix delete button alignment on Edit Tag screen.

The delete button on the Edit Tag screen was misaligned after the form control updates in [61645]. Switch the action buttons row to a flexbox layout for stable alignment.

Follow-up to [61645].

Props mukesh27, tusharbharti, wildworks.
Fixes #65233.

git-svn-id: https://develop.svn.wordpress.org/trunk@62357 602fd350-edb4-49c9-b593-d223f7449a82