Guidelines can have html, so fix the config schema

Problem/Motivation

Since 10.3, config schema allows the html data type. Use that for the guidelines.
See https://www.drupal.org/node/3450544 →

Proposed resolution

Change guidelines in config from string to html.

Remaining tasks

Fix schema.

User interface changes

None.

API changes

Changed guidelines data type from string to html.

Data model changes

Changed guidelines data type from string to html.