Replace boolean values with strings

What does this MR do?

Related #915 (closed). Fixes the error thrown in our Storybook:

Steps to reproduce

1. Go to https://gitlab-org.gitlab.io/gitlab-ui/?path=/story/base-filtered-search--default
2. Clear out the filtered search
3. Choose "Confidential"
4. Choose "=" or "!="
5. Choose "Yes" or "No"

Conformity

• Code review guidelines.
• GitLab UI's contributing guidlines.
• If it changes a Pajamas-compliant component's look & feel, the MR has been reviewed by a UX designer.
• If it changes GitLab UI's documentation guidelines, the MR has been reviewed by a Technical Writer.
• If the MR changes a component's API, integration MR(s) have been opened in the following projects to ensure that the @gitlab/ui package can be upgraded quickly after the changes are released:
  • GitLab: mr_url
  • Customers Portal: mr_url
  • Status Page: mr_url
• Added the ~"component:*" label(s) if applicable.