Bump html-pipeline to 2.7.1

What does this MR do?

Bumps html-pipeline to 2.7.1.

It should be bumped to migrate to rails 5.0 and not introduce a backport of the search_text_nodes method which has been removed in the latest versions of the gem.

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

No.

Why was this MR needed?

Make migration to rails 5.0 smoother.

Screenshots (if relevant)

No.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #24039 (closed)