[development] Minimum MySQL version

[catch]

Since no-one's objected, I've opened an issue here:

http://drupal.org/node/220329

Also - I've seen dropping PostgreSQL 7.4 support mentioned in a couple
of issues. I won't claim to know what changed between 7.4 and 8.x (nor
when support is due to be dropped for 7.4 - it's still getting security
releases at least), but it's been around since 2003 now...