SearchPage entities implicitly require labels: make explicit, and fix migration from D7

Problem/Motivation

This was discovered during 📌 New config schema data type: `required_label` Fixed .

It turns out that SearchPage entities are migrated without labels. It's not clear how migrated search pages appear in the UI, then, since the entity type has no special handling for labels.

Proposed resolution

Ensure that search pages have labels when they are migrated, and then make search page labels required in config schema (using the required_label data type introduced by 📌 New config schema data type: `required_label` Fixed ).

Remaining tasks

Fix the migration first, and ensure the test coverage is up-to-date. Then make the config schema change to match.

User interface changes

None, really.

API changes

Not really.

Data model changes

Search pages will be required to have labels.

Release notes snippet

TBD