Fix Scoped Boards bug filtering by No Milestone

What does this MR do?

don't encode the + in No+Milestone

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

Why was this MR needed?

~bug

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary

• Documentation created/updated

• Tests added for this feature/bug

• Review

  • Has been reviewed by Frontend
  • Has been reviewed by Backend

• 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-qa manual pipeline job)

What are the relevant issue numbers?

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/43317