What happened to MySQL 6 & 7?
According to Dave Stockes, a MySQL Community Manager for Oracle:
(from his blog post: MySQL 8 is coming)


  Years ago, before the Sun Microsystems purchase of MySQL AB, there was a version of MySQL with the number 6. Sadly, it was a bit ambitious and the change of ownership left it to wither. The MySQL Cluster product has been using the 7 series for years. With the new changes for MySQL 8, developers feel they have modified it enough to bump the big number.


Wikipedia article about MySQL also has these events:


  
  Sun Microsystems acquired MySQL AB in 2008.  
  Version 5.1: production release 27 November 2008 (event scheduler, partitioning, plugin API, row-based replication, server log tables)
  MySQL 5.1 and 6.0-alpha showed poor performance when used for data warehousing – partly due to its inability to utilize multiple CPU cores for processing a single query.   
  MySQL Server 6.0.11-alpha was announced on 22 May 2009 as the last release of the 6.0 line. Future MySQL Server development uses a New Release Model. Features developed for 6.0 are being incorporated into future releases.  
  Oracle acquired Sun Microsystems on 27 January 2010.  
  Work on version 6 stopped after the Sun Microsystems acquisition. The MySQL Cluster product uses version 7. The decision was made to jump to version 8 as the next major version number.  
  




And for anyone who wants to see the code, here is a repository: MySQL 6.0 binaries and source code (6.0.11-alpha).