You can use practically any flash with an M6 (so long as they don't have some weird proprietary shoe/contact scheme)--although some are, certainly, easier to use. You can of course use them in manual mode, but most all of them will work fine in Auto mode, too, so long as you match the aperture on your lens with the setting on your flash, and make sure your ISO also matches. You really shouldn't have anything to worry about in terms of damaging your camera's electronics; as v0sh pointed out, digital cameras have far more to fear from high trigger voltages on older flashes than your M6 ever will.
I personally use a small, but quite capable, Metz 34 CS-2.
The big obstacle really is that pesky screw in the front center of the hotshoe, that prevents you from sliding most flashes on all the way (and so may prevent the center firing contact on the flash from connecting with the M6's hotshoe contact). I'm sure it's removable--but then you run the risk of losing/misplacing it; that will bug you, and could lower the resale value of the camera, given how obsessive many Leicaphiles are. (EDIT: it turns out that, on some M models at least, that screw holds in place some important parts; they'll also fall out if you take out the screw.)
The way I get around that is, I once found a PC to hotshoe adapter made by Kaiser, which has a notch in the front of its base that slides right over that screw. When I shoot flash with my M6, I slip the adapter into the hotshoe, plug the end of the cable into the PC sync socket (making sure not to lose the bit of expensive plastic covering it), and then slide my flash or radio trigger into the adapter--I'm actually using the PC sync socket, not the hotshoe, to trigger my flash, even though it's mounted (via the adapter) in the hotshoe. I've attached photos below.
Before you buy a current version, might want check to see if they still come with the notch--I got mine way back in the late 90s, if I recall correctly. But you really don't *need* the notch, because with the adapter, you're going to use the PC sync socket, not the hotshoe to fire the flash--so the adapter doesn't have to go in all the way. Also, Leica flashes designed to be used on M cameras surely I'd think--I don't own one to say certainly--have the notch built-in; I don't know if your CF22 does, as Leica may not have wanted people "cheating" by using one on an M camera. From the photos I can see online, it doesn't look like it does.
You *could*, with a bit of delicate surgery with a drill or a Dremel, grind your own notch in the front of your flash's foot, if you don't want to risk removing and then misplacing that screw in the M6 hotshoe. It's up to you.
