dkan 4.1.0

Several new fixes and small improvements, with two breaking changes, so please read full release notes:

• Add table create event with DataResource (#4723) by mile23
• Metastore Search - check for empty facet from content (#4712) by stefan.korn
• Migrate to opis/json-schema v2 (#4706) by dcgoodwin, dafeder
• Symfony polyfill workaround (#4725) by dafeder
• Frontend module cleanup (#4699) by mile23
• Explain how to avoid losing transitional modules in 3.x in upgrade docs (#4696) by dafeder
• Empty datastore error (#4721)

Breaking change 1: JSON Schema Validator Update

Moving to Opis JSON Schema Library v2 may break validation of your schemas if you have overridden DKAN's out-of-box dataset or other schemas. Be sure to read the full change record for some background and instructions on how to update your schemas to be compatible. You will see update hook and Drupal status messages alerting you to the fact if your schemas are failing after the update.

Breaking change 2: DatabaseTableInterface

This breaking change is much less likely to affect any projects downstream. A new public method has been added to Drupal\dkan_common\Storage\DatabaseTableInterface. If you implement that interface in any custom module code, you will need to add a ::tableIsEmpty() method.