Display "commented" only for commit discussions on merge requests

What does this MR do?

Adjust the discussion header on issues and merge requests to match the design.

What are the relevant issue numbers?

closes https://gitlab.com/gitlab-org/gitlab-ce/issues/53681

closes https://gitlab.com/gitlab-org/gitlab-ce/issues/53950

Screenshots

issue

Before	After

merge request

Before	After

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• 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