Bump govuk_design_system_formbuilder from 5.5.0 to 5.6.0

Bumps [govuk_design_system_formbuilder](https://github.com/x-govuk/govuk-form-builder) from 5.5.0 to 5.6.0.
- [Release notes](https://github.com/x-govuk/govuk-form-builder/releases)
- [Commits](x-govuk/govuk-form-builder@v5.5.0...v5.6.0)

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

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

Gemfile.lock

@@ -226,7 +226,7 @@ GEM
       html-attributes-utils (~> 1.0.0, >= 1.0.0)
       pagy (>= 6, < 10)
       view_component (>= 3.9, < 3.15)
-    govuk_design_system_formbuilder (5.5.0)
+    govuk_design_system_formbuilder (5.6.0)
       actionview (>= 6.1)
       activemodel (>= 6.1)
       activesupport (>= 6.1)