Replace collective.volto.formsupport with plone.formblock (#246)

[ericof]

Summary

Adopt plone.formblock as the form solution for kitconcept.core and migrate away from collective.volto.formsupport.

plone.formblock is the upstream successor to collective.volto.formsupport (heading toward Plone core in 6.3), so this moves the distribution onto the supported path.

Changes (backend)

• Add the plone.formblock>=1.0.0a2 dependency.
• Install plone.formblock:default on new sites and stop installing collective.volto.formsupport:default.
• Unconfigure the form_data soup catalog utility and the form-data REST service from collective.volto.formsupport (via z3c.unconfigure) so they do not clash with the equivalents registered by plone.formblock.
• Add upgrade step v20260619001 to uninstall collective.volto.formsupport and install plone.formblock on existing sites.
• Update the setup tests to assert plone.formblock is installed and collective.volto.formsupport is uninstalled.

Note: the collective.volto.formsupport distribution is intentionally kept as a dependency for now, as we need to uninstall it from existing sites. It will be dropped in a follow-up once the migration has settled. Also we use the unconfigure directive to resolve ZCA registration classes.

Issue

Relates to kitconcept/distribution-kitconcept-intranet#246 — https://gitlab.kitconcept.io/kitconcept/distribution-kitconcept-intranet/-/issues/246

Test plan

• make test — full backend suite: 140 passed, 3 skipped.
• ruff lint + format clean; zpretty --check clean.