Config schema error after 1.1.2 update [#3551604]

Problem/Motivation

Something seems not right with the config schema which was introduced in #3547314: Configuration interface.

The Config Inspector module also confirms the same issue.

>  [notice] Update started: markdownify_update_10003
>  [warning] Schema errors for markdownify.settings with the following errors: markdownify.settings:supported_entities.node missing schema, markdownify.settings:supported_entities.taxonomy_term missing schema. These errors mean there is configuration that does not comply with its schema. This is not a fatal error, but it is recommended to fix these issues. For more information on configuration schemas, check out <a href="https://www.drupal.org/docs/drupal-apis/configuration-api/configuration-schemametadata">the documentation</a>.
>  [notice] Added default "supported_entities" configuration for supported entity types: node, taxonomy_term.
>  [notice] Update completed: markdownify_update_10003

Steps to reproduce

Update a site from 1.1.1 to 1.1.2.

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Issue fork markdownify-3551604

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