Clarify that innodb_stats_on_metadata is actual only in MySQL<=5.6

[paskal]

Sources:

• https://www.percona.com/blog/innodb_stats_on_metadata-slow-queries-information_schema/
• https://dev.mysql.com/doc/refman/5.7/en/innodb-parameters.html#sysvar_innodb_stats_on_metadata
• https://dev.mysql.com/doc/refman/8.0/en/innodb-parameters.html#sysvar_innodb_stats_on_metadata

[jmrenouard]

Thanks @paskal

[paskal]

I just realised what I did wrong: it's not "in MySQL 5.6 and lower" but "enabled by default in MySQL 5.6 and lower", I'll prepare another PR.

[paskal]

Or, actually, should we drop it altogether? It's a strange one-off historic artefact which describes a problem which was actual before 2015, and the tool itself reports if that value is in the ON state anyway.