Banzai::Filter::ExternalLinkFilter use XPath

What does this MR do?

Use XPath expresions to search in html documents instead CSS ones

Why was this MR needed?

To try to speed up the Banzai Filter

What are the relevant issue numbers?

Closes #18582 (closed)

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together