Address D9 coding standards and deprecations

.../core.entity_form_display.eventseries.default.default.yml

config/install/recurring_events.eventseries_type.default.yml

config/schema/recurring_events.schema.yml

js/recurring_events_create_form.js

js/recurring_events_date_form.js

modules/recurring_events_registration/README.md

...l/core.entity_form_display.registrant.default.default.yml

...stall/recurring_events_registration.registrant.config.yml

...recurring_events_registration.registrant_type.default.yml

...events_registration/recurring_events_registration.api.php

...gistration/recurring_events_registration.links.action.yml

..._events_registration/recurring_events_registration.module

...ts_registration/recurring_events_registration.routing.yml

...s_registration/recurring_events_registration.services.yml

...nts_registration/recurring_events_registration.tokens.inc

modules/recurring_events_registration/src/AccessHandler.php

...ents_registration/src/Controller/RegistrantController.php

...s/recurring_events_registration/src/Entity/Registrant.php

...es/recurring_events_registration/src/Form/ContactForm.php

...recurring_events_registration/src/Form/RegistrantForm.php