Mention in comments wrongly attributed to the last document content editor

 Reproduction steps:

• Create 3 users U1, U2, U3
• Create a page with U1
• Add a comment mentioning U2 with U3

Expected results:

• U2 receives a mention attributed to U3

Actual results:

• U2 receives a mention attributed to U1 (the last author of the page content)

This issue is caused by XWIKI-19415 where the deprecated com.xpn.xwiki.doc.XWikiDocument#getAuthorReference has been replaced by org.xwiki.model.document.DocumentAuthors#getEffectiveMetadataAuthor