Breaking: #68243 - Move not used FrontendDocumentTemplate 

See forge#68243

Description 

Move unused FrontendDocumentTemplate to ext:compatibility6.

Impact 

Installations still using FrontendDocumentTemplate require ext:compatibility6 to be installed.

Affected Installations 

Installations still using FrontendDocumentTemplate.

Migration 

Install ext:compatibility6 or adapt the code to not use the FrontendDocumentTemplate functionality.

Adapting the code is highly recommended.