Bump govuk_design_system_formbuilder from 5.13.0 to 6.0.0

Bumps [govuk_design_system_formbuilder](https://github.com/x-govuk/govuk-form-builder) from 5.13.0 to 6.0.0.
- [Release notes](https://github.com/x-govuk/govuk-form-builder/releases)
- [Commits](x-govuk/govuk-form-builder@v5.13.0...v6.0.0)

---
updated-dependencies:
- dependency-name: govuk_design_system_formbuilder
  dependency-version: 6.0.0
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

Gemfile.lock

@@ -270,7 +270,7 @@ GEM
       rake (>= 10.0)
     globalid (1.3.0)
       activesupport (>= 6.1)
-    govuk_design_system_formbuilder (5.13.0)
+    govuk_design_system_formbuilder (6.0.0)
       actionview (>= 6.1)
       activemodel (>= 6.1)
       activesupport (>= 6.1)