Deprecation: #EXTBLOG-57 - Add author relations

• See https://jira.typo3.com/browse/EXTBLOG-57
• See https://jira.typo3.com/browse/TE-21

Description

The author filed has been marked as deprecated. Use the new author relations.

See Feature: #EXTBLOG-57 - Add author relations

Impact

The “author” field can still be used, but should be replaced by the new author record.