The data is on the wiki, it's just a bit hard to find, primarily because the strength value to remove the penalty varies.
From the Strength entry, the strength required to wear heavy armour is:
STR = EV penalty times 3
It is the EV penalty that varies, hence why the strength value required to wear heavy armour also varies. There are a number of factors that are used to calculate the EV penalty, and the precise calculation does change between versions, so that the wiki is often out of date. Regardless, it is generally the case that your dexterity actually plays a part in the calculation, such that a character with a low dexterity will need more strength to remove the penalty and a character with high dexterity will need less strength.
Due to this, and the part dexterity plays in attack rolls, I tend to keep my strength and dexterity identical when playing a fighter character, unless I am playing a character that starts with a massive strength bonus (such as a Mountain Dwarf Fighter), in which case I just put a few points in dexterity so that it doesn't lag too far behind my strength.
One final note - the message does not indicate that you cannot wear heavy armour, it just indicates that you won't be able to use it effectively if you do wear it. In other words, your ability to block attacks with your armour will not be as good as if you had a higher strength. No character in Crawl is prevented from using any piece of equipment.