[1.6.x] Updated MySQL links to version 5.6.

Backport of fb4f3e04b1 from master
[1.6.x] Fixed #23149 -- Clarified note on HTTPOnly in cookie-based session docs

Backport of e26366da44bb343e7a95d01ff0dd18b8026c2802 from master.
[1.6.x] Fixed #23154 -- Removed reference to deprecated run_gunicorn command.

Thanks frewsxcv for the report.

Backport of 75f2c65cb6 from master