Remove card-title in card-header

What does this MR do?

Remove all .card-title under .card-header from BS 4.0 card components.

Are there points in the code the reviewer needs to double check?

grep card-title to get 0 matches.

Why was this MR needed?

• To simply follow BS4' philosophy
• To remove GitLab's workaround for .card-title
  • to improve contributor's experience

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [n/a] Documentation created/updated
• [n/a] API support added
• Tests added for this feature/bug
• Conform by the code review guidelines
  • [n/a] Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • [n/a] Has been reviewed by a Backend maintainer
  • [n/a] Has been reviewed by a Database specialist
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• [n/a] Internationalization required/considered
• [n/a] End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #46884 (closed)