Deprecation: #77934 - Deprecate tt_content field select_key 

See forge#77934

Description 

The field select_key of the table tt_content is not used in the core and has been removed.

Impact 

The field select_key is not available by default anymore.

Affected Installations 

All installations and extensions using the field select_key of the table tt_content.

Migration 

Install the extension compatibility7 to enable the field again.