Rename 'All issues' to 'Open issues' in Add issues modal

What does this MR do?

It renames All issues to Open issues in the Add issues modal on the Issues Board page.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #29617 (closed).